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Prevention and Control of Influenza
Recommendations of the Advisory Committee on Immunization
Practices (ACIP), 2007
Prepared by
Anthony E. Fiore, MD1
David K. Shay, MD1
Penina Haber, MPH3
John K. Iskander, MD3
Timothy M. Uyeki, MD1
Gina Mootrey, DO2
Joseph S. Bresee, MD1
Nancy J. Cox, PhD1
1Influenza Division, National Center for Immunization and Respiratory Diseases
2Immunization Services Division, National Center for Immunization and Respiratory Diseases
3Immunization Safety Office, Office of the Director
The material in this report originated in the National Center for Immunization and Respiratory Diseases, Anne Schuchat, MD, Director; the
Influenza Division, Nancy Cox, PhD, Director; and the Immunization Services Division, Lance Rodewald, MD, Director.
Corresponding preparer: Anthony Fiore, MD, Influenza Division, National Center for Immunization and Respiratory Diseases, CDC, 1600
Clifton Road, NE, MS A-20, Atlanta, GA 30333. Telephone: 404-639-2552; Fax: 404-639-2334; E-mail: afiore@cdc.gov.
Summary
This report updates the 2006 recommendations by CDC's Advisory Committee on Immunization Practices (ACIP)
regarding the use of influenza vaccine and antiviral agents
(CDC. Prevention and control of influenza: recommendations of
the Advisory Committee on Immunization Practices [ACIP]. MMWR 2006;55[No.
RR-10]). The groups of persons for whom vaccination is recommended and the antiviral medications recommended for chemoprophylaxis or treatment (oseltamivir
or zanamivir) have not changed. Estimated vaccination coverage remains <50% among
certain groups for whom routine annual vaccination is recommended, including young children and adults with risk factors for influenza complications,
health-care personnel (HCP), and pregnant women. Strategies to improve vaccination coverage, including use of reminder/recall systems
and standing orders programs, should be implemented or expanded. The 2007 recommendations include new and
updated information. Principal updates and changes include 1) reemphasizing the importance of administering 2 doses of vaccine to
all children aged 6 months--8 years if they have not been vaccinated previously at any time with either
live, attenuated influenza vaccine (doses separated by
>6 weeks) or trivalent inactivated influenza vaccine (doses separated by
>4 weeks), with single annual doses in subsequent years;
2) recommending that children aged 6 months--8 years who received only 1 dose in their first year
of vaccination receive 2 doses the following year, with single annual doses in subsequent
years; 3) highlighting a previous recommendation that all persons, including school-aged children, who want to reduce the risk of becoming ill with influenza or
of transmitting influenza to others should be vaccinated;
4) emphasizing that immunization providers should offer influenza
vaccine and schedule immunization clinics throughout the influenza season;
5) recommending that health-care facilities consider the level
of vaccination coverage among HCP to be one measure of a patient safety quality program and implement policies to encourage
HCP vaccination (e.g., obtaining signed statements from
HCP who decline influenza vaccination); and 6) using the 2007--2008
trivalent vaccine virus strains A/Solomon Islands/3/2006 (H1N1)-like (new for this season), A/Wisconsin/67/2005 (H3N2)-like, and
B/Malaysia/2506/2004-like antigens. This report and other information are available at CDC's influenza website
(http://www.cdc.gov/flu). Updates or supplements to these recommendations (e.g., expanded age or risk group indications for currently licensed
vaccines) might be required. Immunization providers should be alert to announcements of recommendation updates and should check
the CDC influenza website periodically for additional information.
Introduction
In the United States, annual epidemics of influenza occur typically during the late fall and winter seasons; an annual
average of approximately 36,000 deaths during 1990--1999 and 226,000 hospitalizations during 1979--2001 have been
associated
with influenza epidemics (1,2). Influenza viruses can cause disease among persons in any age group
(3--5), but rates of infection are highest among children. Rates of serious illness and death are
highest among persons aged >65 years, children
aged <2 years, and persons of any age who have medical conditions that place them at increased risk for complications from
influenza (3,6--8).
Influenza vaccination is the most effective method for preventing influenza virus infection and its potentially
severe complications. Influenza immunization efforts are focused primarily on providing vaccination to persons at risk
for influenza complications and to contacts of these persons
(Box). Influenza vaccine may be administered to any person
aged >6 months to reduce the likelihood of becoming ill with
influenza or of transmitting influenza to others; if vaccine supply
is limited, priority for vaccination is typically assigned to persons in specific groups and of specific ages who are, or are
contacts of, persons at higher risk for influenza complications. Trivalent inactivated influenza vaccine (TIV) may be used for any
person aged >6 months, including those with high-risk conditions. Live, attenuated influenza vaccine (LAIV) currently is
approved only for use among healthy, nonpregnant persons aged 5--49 years. Because influenza viruses undergo frequent
antigenic change (i.e., antigenic drift), persons recommended for vaccination must receive an annual vaccination against the
influenza viruses currently in circulation. Although vaccination coverage has increased in recent years for many groups recommended
for routine vaccination, coverage remains unacceptably low, and strategies to improve vaccination coverage, including use
of reminder/recall systems and standing orders programs, should be implemented or expanded.
Antiviral medications are an adjunct to vaccination and are effective when administered as treatment and when used
for chemoprophylaxis after an exposure to influenza virus. Oseltamivir and zanamivir are the only antiviral medications
currently recommended for use in the United States. Resistance to oseltamivir or zanamivir remains rare. Amantadine or
rimantidine should not be used for the treatment or prevention of influenza in the United States until evidence of susceptibility to
these antiviral medications has been reestablished among circulating influenza A viruses.
Methods
CDC's Advisory Committee on Immunization Practices (ACIP) provides annual recommendations for the prevention
and control of influenza. The ACIP Influenza Vaccine Working
Group* meets monthly throughout the year to discuss
newly published studies, review current guidelines, and consider potential revisions to the recommendations. As they
review the annual recommendations for ACIP consideration, members of the Working Group consider a variety of issues,
including vaccine effectiveness, safety and coverage in groups recommended for vaccination, feasibility, cost-effectiveness,
and anticipated vaccine supply. Working Group members also request periodic updates on vaccine and antiviral
production, supply, safety and efficacy from vaccinologists, epidemiologists and manufacturers. State and local immunization
program representatives are consulted. Influenza surveillance and antiviral resistance data were obtained from CDC's
Influenza Division. The Vaccines and Related Biological Products
Advisory Committee of the Food and Drug Administration
(FDA) selects the viral strains to be used in the annual trivalent influenza vaccines.
Published, peer-reviewed studies identified through literature searches are the primary source of data used in making
these recommendations. Among studies discussed or cited, those of greatest scientific quality and those that measured
influenza-specific outcomes were the most influential during the development of these recommendations. For example,
population-based estimates that use outcomes associated with laboratory-confirmed influenza virus infection contribute the most
specific data for estimates of influenza burden. The best evidence for vaccine or antiviral efficacy and effectiveness studies comes
from randomized controlled trials that assess laboratory-confirmed influenza infections as an outcome measure and consider
factors such as timing and intensity of influenza circulation and degree of match between vaccine strains and wild circulating
strains (9,10). Randomized, placebo-controlled
trials cannot be performed in populations for which vaccination already
is recommended, but observational studies that assess outcomes associated with laboratory-confirmed influenza infection
can provide important vaccine effectiveness data. Randomized, placebo-controlled clinical trials are the best source of vaccine
and antiviral safety data for common adverse events; however, such studies do not have the power to identify rare but
potentially serious adverse events. The frequency of rare adverse events that might be associated with vaccination or antiviral treatment
is best assessed by retrospective reviews of computerized medical records from large linked clinical databases, with chart
review for persons who are identified as having a potential adverse event after vaccination
(11,12). Vaccine coverage data from a nationally representative, randomly selected population that includes verification of vaccination through health-care
record
review is superior to coverage data derived from limited populations or without verification of immunization but is
rarely available for older children or adults
(13). Finally, studies that assess immunization program practices that improve
vaccination coverage are most influential in formulating recommendations if the study design includes a nonintervention
comparison group. In cited studies that included statistical comparisons, a difference was considered to be statistically significant if the
p-value was <0.05 or the 95% confidence interval (CI) around an estimate of effect allowed rejection of the null hypothesis
(i.e., no effect).
These recommendations were presented to the full ACIP and approved in February 2007. Modifications were made to
the ACIP statement during the subsequent review process at CDC to update and clarify wording in the document. Data
presented in this report were current as of June 27, 2007. Further updates, if needed, will be posted at CDC's influenza website
(http://www.cdc.gov/flu).
Primary Changes and Updates in the Recommendations
The 2007 recommendations include six principal changes or updates:
- ACIP reemphasizes the importance of administering
2 doses of vaccine to all children aged 6 months--8 years if they
have not been vaccinated previously at any time with either LAIV (doses separated by
>6 weeks) or TIV (doses separated by >4 weeks), on the basis of accumulating data indicating that 2 doses are required for protection in these children (see
Vaccine Efficacy, Effectiveness, and Safety).
- ACIP recommends that children aged 6 months--8 years who received only 1 dose in their first year of vaccination
receive 2 doses the following year (see Vaccine Efficacy, Effectiveness, and Safety).
- ACIP reiterates a previous recommendation that all
persons, including school-aged children, who want to reduce the
risk of becoming ill with influenza or of transmitting influenza to others should be vaccinated (see Box and
Recommendations for Using TIV and LAIV During the 2007--08
Influenza Season).
- ACIP emphasizes that immunization providers should offer influenza vaccine and schedule immunization
clinics throughout the influenza season (see Timing of Vaccination).
- ACIP recommends that health-care administrators consider the level of vaccination coverage among
health-care personnel (HCP) to be one measure of a patient safety quality program and implement policies to
encourage HCP vaccination (e.g., obtaining signed statements from HCP who decline influenza vaccination)
(see Additional Information Regarding Vaccination of Specific Populations).
- The 2007--2008 trivalent vaccine strains are A/Solomon Islands/3/2006 (H1N1)-like (new for this season),
A/Wisconsin/67/2005 (H3N2)-like, and B/Malaysia/2506/2004-like viruses. (see Recommendations for Using TIV and LAIV
During the 2007--08 Influenza Season).
Background and Epidemiology
Biology of Influenza
Influenza A and B are the two types of influenza viruses that cause epidemic human disease
(14). Influenza A viruses are categorized into subtypes on the basis of two surface antigens: hemagglutinin and neuraminidase. Currently
circulating influenza B viruses are separated into two distinct genetic lineages but are not categorized into subtypes. Since 1977,
influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have circulated globally. In certain recent
years, influenza A (H1N2) viruses that probably emerged
after genetic reassortment between human A (H3N2) and
A (H1N1) viruses also have circulated. Both influenza A subtypes and B viruses are further separated into groups on the basis of
antigenic similarities. New influenza virus variants result from frequent antigenic change (i.e., antigenic drift) resulting from
point mutations that occur during viral replication. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses.
Immunity to the surface antigens, particularly the hemagglutinin, reduces the likelihood of infection
(15). Antibody against one influenza virus type or subtype confers limited or no protection against another type or subtype of influenza
virus. Furthermore, antibody to one antigenic type or subtype of influenza virus might not protect against infection with a
new antigenic variant of the same type or subtype
(16). Frequent emergence of antigenic variants through antigenic drift is
the
virologic basis for seasonal epidemics as well as the reason for annually reassessing the need to change one or more of
the recommended strains for influenza vaccines.
More dramatic changes, or antigenic shifts, occur less frequently and can result in the emergence of a novel influenza A
virus with the potential to cause a pandemic. Antigenic shift occurs when a new subtype of influenza A virus emerges
(14). New influenza A subtypes have the potential to cause a pandemic when they are demonstrated to be able to cause human illness
and demonstrate efficient human-to-human transmission, in the setting of little or no previously existing immunity
among humans.
Clinical Signs and Symptoms of Influenza
Influenza viruses are spread from person to person primarily through large-particle respiratory droplet transmission
(e.g., when an infected person coughs or sneezes near a susceptible person)
(14). Transmission via large-particle droplets
requires close contact between source and recipient persons, because droplets do not remain suspended in the air and generally
travel only a short distance (<1 meter) through the air. Contact with respiratory-droplet contaminated surfaces is another
possible source of transmission. Airborne transmission (via small-particle residue
[<5µm] of evaporated droplets that might
remain suspended in the air for long periods of time) also is thought to be possible, although data supporting airborne
transmission are limited (17--20). The typical incubation period for influenza is 1--4 days (average: 2 days)
(21). Adults can be infectious from the day before symptoms begin through approximately 5 days after illness onset. Young children also might shed
virus several days before illness onset, and children can be infectious for
>10 days after onset of symptoms.
Severely immunocompromised persons can shed virus for weeks or months
(22--25).
Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and
symptoms (e.g., fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis)
(26). Among children, otitis media, nausea, and vomiting also are commonly reported with influenza illness
(27--29). Uncomplicated influenza illness
typically resolves after 3--7 days for the majority of persons,
although cough and malaise can persist for >2 weeks. However,
influenza virus infections can cause primary influenza viral pneumonia; exacerbate underlying medical conditions (e.g., pulmonary
or cardiac disease); lead to secondary bacterial pneumonia, sinusitis, or otitis; or contribute to coinfections with other viral
or bacterial pathogens (30--32). Young children with influenza virus infection might have initial symptoms mimicking
bacterial sepsis with high fevers (31--34), and febrile seizures have been reported in 6%--20% of children hospitalized with
influenza virus infection (28,31,35). Population-based studies among hospitalized children with laboratory-confirmed influenza
have demonstrated that although the majority of hospitalizations are brief
(<2 days), 4%--11% of children hospitalized
with laboratory-confirmed influenza required treatment in the intensive care unit, and 3% required mechanical ventilation
(31,33). Among 1,308 hospitalized children in one study, 80% were aged <5 years, and 27% were aged <6 months
(31). Influenza virus infection also has been uncommonly associated with encephalopathy, transverse
myelitis, myositis, myocarditis, pericarditis, and Reye syndrome
(28,30,36,37).
Respiratory illnesses caused by influenza virus infection are difficult to distinguish from illnesses caused by other
respiratory pathogens on the basis of signs and symptoms alone. Sensitivity and predictive value of clinical definitions can vary,
depending on the degree of circulation of other respiratory pathogens and the level of influenza activity
(38). Among generally healthy older adolescents and adults living in areas with confirmed influenza virus circulation, estimates of the positive predictive
value of a simple clinical definition of influenza (cough and fever) for laboratory-confirmed influenza infection have varied
(range: 79%--88%) (39,40).
Young children are less likely to report typical influenza symptoms (e.g., fever and cough). In studies conducted
among children aged 5--12 years, the positive predictive value of
fever and cough together was 71%--83%, compared with
64% among children aged <5 years
(41). In one large, population-based surveillance study in which all children with fever
or symptoms of acute respiratory tract infection were tested for influenza, 70% of hospitalized children aged <6 months
with laboratory-confirmed influenza were reported to have fever and cough, compared with 91% of hospitalized children aged
6 months--5 years. Among children with laboratory-confirmed influenza infections, only 28% of those hospitalized and 17%
of those treated as outpatients had a discharge diagnosis of influenza
(34). A study of older nonhospitalized
patients determined that the presence of fever, cough, and acute onset had a positive predictive value of only 30% for influenza
(42). Among hospitalized older patients with chronic cardiopulmonary disease, a combination of fever, cough, and illness of <7
days was 53% predictive for confirmed influenza infection
(43). The absence of symptoms of influenza-like illness (ILI) does
not
effectively rule out influenza; among hospitalized adults with laboratory-confirmed infection, only 51% had typical
ILI symptoms of fever plus cough or sore throat
(44). A study of vaccinated older persons with chronic lung disease reported
that cough was not predictive of laboratory-confirmed influenza virus infection, although having both fever or feverishness
and myalgia had a positive predictive value of 41%
(45). These results highlight the challenges of identifying
influenza illness in the absence of laboratory confirmation and indicate that the diagnosis of influenza should be considered in any patient
with respiratory symptoms or fever during influenza
season.
Hospitalizations and Deaths from Influenza
In the United States, annual epidemics of influenza typically occur during the fall or winter months, but the peak
of influenza activity can occur as late as April or May (Table 1).
Influenza-related hospitalizations or deaths can result from
the direct effects of influenza virus infection or from complications due to underlying cardiopulmonary conditions and
other chronic diseases. Studies that have measured rates of a clinical outcome without a laboratory confirmation of influenza
virus infection (e.g., respiratory illness requiring hospitalization during influenza season) to assess the effect of influenza can
be difficult to interpret because of circulation of other respiratory pathogens (e.g., respiratory syncytial virus) during the
same time as influenza viruses (46--48).
During seasonal influenza epidemics from 1979--1980 through 2000--2001, the estimated annual overall number
of influenza-associated hospitalizations in the United States ranged from approximately 55,000 to 431,000 per epidemic
(mean: 226,000); the estimated annual number of deaths
attributed to influenza ranged from 8,000 to 68,000 per epidemic
(mean: 34,000) (1,2). Since the 1968 influenza A (H3N2) virus pandemic, the number of influenza-associated
hospitalizations typically has been greater during seasonal
influenza epidemics caused by type A (H3N2) viruses than during seasons in
which other influenza virus types or subtypes have predominated
(49). In the United States, the number of
influenza-associated deaths has increased since 1990. This increase has been attributed in part to the substantial increase in the number of
persons aged >65 years, who are at increased risk for death from influenza complications
(50). In one study, an average of approximately 19,000
influenza-associated pulmonary and circulatory deaths per influenza season occurred during
1976--1990, compared with an average of approximately 36,000 deaths per season during 1990--1999
(1). In addition, influenza A (H3N2) viruses, which have been associated with higher mortality
(51), predominated in 90% of influenza seasons
during 1990--1999, compared with 57% of seasons during 1976--1990
(1).
Influenza viruses cause disease among persons in all age groups
(3--5). Rates of infection are highest among children, but
the risks for complications, hospitalizations, and deaths from influenza are higher among persons aged
>65 years, young children, and persons of any age who have medical conditions that place them at increased risk for complications from influenza
(1,3,6--8,52--55). Estimated rates of
influenza-associated hospitalizations and deaths varied substantially by age group in
studies conducted during different influenza epidemics
(Table 2). During 1990--1999, estimated rates of
influenza-associated pulmonary and circulatory deaths per 100,000 persons were 0.4--0.6 among persons aged 0--49 years, 7.5 among
persons aged 50--64 years, and 98.3 among persons aged
>65 years (1).
Children
Rates of influenza-associated hospitalization are higher among young
children than among older children when influenza
viruses are in circulation and similar to rates for other groups considered at high risk for
influenza-related complications (49,56--61), including persons aged
>65 years (57,58). During 1979--2001, the estimated rate of
influenza-associated hospitalizations in the United States among children aged <5 years was approximately 108 hospitalizations
per 100,000 person-years (2). Recent population-based
studies that have measured hospitalization rates for
laboratory-confirmed influenza in young children have been consistent with studies that analyzed medical discharge data
(29,32--34,60). Annual hospitalization rates for laboratory-confirmed influenza decrease with increasing age, ranging from 240--720 per
100,000 children aged <6 months to approximately 20 per 100,000 children aged 2--5 years
(34). Estimated hospitalization rates for young children with high-risk medical conditions are approximately 250--500 per 100,000 children
(53,55) (Table 2).
Influenza-associated deaths are uncommon among children but represent a substantial proportion of
vaccine-preventable deaths. An estimated annual average of 92 influenza-related deaths (0.4 deaths per 100,000 persons) occurred among
children aged <5 years during the 1990s, compared with 32,651 deaths (98.3 per 100,000 persons) among adults aged
>65 years (1). Of 153 laboratory-confirmed influenza-related
pediatric deaths reported during the 2003--04 influenza season, 96
(63%)
deaths were of children aged <5 years and 61 (40%) of children aged <2 years. Among the 149 children who died and
for whom information on underlying health status was available, 100 (67%) did not have an underlying medical condition
that was an indication for vaccination at that time
(62). In California during the 2003--04 and 2004--05 influenza seasons,
51% of children with laboratory-confirmed influenza who died and 40% of those who required admission to an intensive care
unit had no underlying medical conditions
(63). These data indicate that although deaths are more common among children
with risk factors for influenza complications, the majority of pediatric deaths occur among children of all age groups with
no known high-risk conditions. The annual number of deaths among children
reported to CDC for the past four influenza seasons has ranged from 44 during 2004--2005 to 67 during 2006--2007 (CDC, unpublished data, 2007).
Adults
Hospitalization rates during influenza season are substantially increased for persons aged
>65 years. One retrospective analysis based on data from medical records collected during 1996--2000 estimated that the risk during influenza
season among persons aged >65 years with underlying conditions that put them at risk for influenza-related complications (i.e.,
one of more of the conditions listed as indications for vaccination) was approximately 56 influenza-associated hospitalizations
per 10,000 persons, compared with approximately 19 per 10,000 healthy elderly persons. Persons aged 50--64 years
with underlying medical conditions also were at substantially increased risk for hospitalizations during influenza season,
compared with healthy adults aged 50--64 years. No increased risk for influenza-associated hospitalizations was demonstrated
among healthy adults aged 50--64 years or among those aged 19--49 years, regardless of underlying medical conditions
(52). During 1976--2001, an estimated yearly average of 32,651 (90%) influenza-related deaths occurred among adults aged
>65 years (1). Risk for
influenza-associated death was highest among the oldest elderly, with persons aged
>85 years 16 times more likely to die from an influenza-associated illness than persons aged 65--69 years
(1).
Limited information is available regarding the frequency and severity of influenza illness among persons with
human immunodeficiency virus (HIV) infection
(64,65). However, a retrospective study of young and middle-aged
women enrolled in Tennessee's Medicaid program determined that the attributable risk for cardiopulmonary hospitalizations
among women with HIV infection was higher during influenza seasons than it was either before or after
influenza was circulating. The risk for hospitalization was higher for HIV-infected women than it was for women with other underlying
medical conditions (66). Another study estimated
that the risk for influenza-related death was 94--146
deaths per 100,000 persons with acquired immunodeficiency syndrome (AIDS), compared with 0.9--1.0 deaths per 100,000 persons aged 25--54
years and 64--70 deaths per 100,000 persons aged
>65 years (67). Influenza symptoms might be prolonged and the risk
for complications from influenza increased for certain
HIV-infected persons (68--70).
Influenza-associated excess deaths among pregnant women were reported during the pandemics of 1918--1919 and
1957--1958 (71--74). Case reports and several epidemiologic studies also indicate that pregnancy can increase the risk for
serious medical complications of influenza
(75--80). The majority of recent studies that have attempted to assess the effect
of influenza on pregnant women have measured changes in excess hospitalizations for respiratory illness during influenza
season but not laboratory-confirmed influenza hospitalizations. Pregnant women have an increased number of medical visits
for respiratory illnesses during influenza season compared with nonpregnant women
(81). Hospitalized pregnant women with respiratory illness during influenza season have increased lengths of stay compared with hospitalized pregnant women
without respiratory illness. For example, rates of hospitalization for respiratory illness were twice as common during
influenza season (82). A retrospective cohort study of approximately 134,000 pregnant women conducted in Nova Scotia during
1990--2002 compared medical record data for pregnant women to data from the same women during the year before pregnancy.
Among pregnant women, 0.4% were hospitalized and 25% visited a clinician during pregnancy for a respiratory illness. The rate
of third-trimester hospital admissions during the influenza season was five times higher than the rate during the influenza
season in the year before pregnancy and more than twice as high as the rate during the noninfluenza season. An excess of
1,210 hospital admissions in the third trimester per 100,000 pregnant women with comorbidities and 68 admissions per
100,000 women without comorbidities was reported
(83). In one study, pregnant women with respiratory hospitalizations did not
have an increase in adverse perinatal outcomes or delivery complications
(84), but they did have an increase in
delivery complications in another study (82). However, infants born to women with laboratory-confirmed influenza during
pregnancy do not have higher rates of low birth weight, congenital abnormalities, or low Apgar scores compared with infants born
to uninfected women (79,85).
Options for Controlling Influenza
The most effective strategy for reducing the effect of influenza is annual vaccination. Strategies that focus on
providing routine vaccination to persons at higher risk for influenza complications have long been recommended, although
coverage among the majority of these groups remains low. Routine vaccination of certain persons (e.g., children and HCP) who serve
as a source of influenza virus transmission might provide additional protection to persons at risk for influenza complications
and reduce the overall influenza burden. Antiviral drugs used for chemoprophylaxis or treatment of influenza are adjuncts
to vaccine but are not substitutes for annual vaccination. Nonpharmacologic interventions (e.g., advising frequent
handwashing and improved respiratory hygiene) are reasonable and inexpensive; these strategies have been demonstrated to
reduce respiratory diseases (86) but have not been studied adequately to determine if they reduce transmission of influenza
virus. Similarly, few data are available to assess the effects of community-level respiratory disease mitigation strategies (e.g.,
closing schools, avoiding mass gatherings, or using masks) on reducing influenza virus transmission during typical seasonal
influenza epidemics (87,88).
Influenza Vaccine Efficacy, Effectiveness, and Safety
Evaluating Influenza Vaccine Efficacy and Effectiveness Studies
The efficacy (i.e., prevention of illness among vaccinated persons in controlled trials) and effectiveness (i.e., prevention
of illness in vaccinated populations) of influenza vaccines
depend primarily on the age and immunocompetence of the
vaccine recipient, the degree of similarity between the viruses in the vaccine and those in circulation, and the
outcome being measured. Influenza vaccine efficacy and effectiveness studies typically have multiple possible outcome
measures, including the prevention of medically attended acute respiratory illness (MAARI), prevention of
laboratory-confirmed influenza virus illness, prevention of influenza or pneumonia-associated hospitalizations or deaths, seroconversion to
vaccine strains, or prevention of seroconversion to circulating influenza virus strains. Efficacy or effectiveness for specific
outcomes such as laboratory-confirmed influenza typically will be higher than for less specific outcomes such as MAARI
because the causes of MAARI include infections with other pathogens that influenza vaccination would not be expected to prevent
(89). Observational studies that compare
less-specific outcomes among vaccinated populations to those among
unvaccinated populations are subject to biases that are difficult to control for during analyses. For example, an
observational study that determines that influenza vaccination reduces overall
mortality might be biased if healthier persons
in the study are more likely to be vaccinated
(90). Randomized controlled trials that measure
laboratory-confirmed influenza virus infections as the
outcome are the most persuasive evidence of vaccine efficacy, but such trials cannot be conducted ethically among groups recommended
to receive vaccine annually.
Influenza Vaccine Composition
Both LAIV and TIV contain strains of influenza viruses that are antigenically equivalent to the annually
recommended strains: one influenza A (H3N2) virus, one influenza A (H1N1) virus, and one influenza B virus. Each year, one or more
virus strains might be changed on the basis of global surveillance for influenza viruses and the emergence and spread of new
strains. Only the H1N1 strain was changed for the recommended vaccine for the 2007--08 influenza season, compared with
the 2006--07 season (see Recommendations for Using TIV and LAIV During the 2007--08 Influenza Season). Viruses for
both types of currently licensed vaccines are grown in eggs. Both vaccines are administered annually to provide optimal
protection against influenza virus infection (Table 3) Both
TIV and LAIV are widely available in the United States. Although both
types of vaccines are expected to be effective, the vaccines differ in several aspects (Table 3)
Major Differences Between TIV and LAIV
During the preparation of TIV, the vaccine viruses are made noninfectious (i.e., inactivated or killed)
(91). Only subvirion and purified surface antigen preparations of TIV (often
referred to as "split" and subunit vaccines, respectively) are available
in the United States. TIV contains killed viruses and thus cannot cause influenza. LAIV contains live, attenuated viruses
and therefore has the potential to produce mild signs or symptoms related to attenuated influenza virus infection. LAIV
is
administered intranasally by sprayer, whereas TIV is administered intramuscularly by injection. LAIV is currently
approved only for use among healthy persons aged 5--49 years; TIV is approved for use among persons aged
>6 months, including those who are healthy and those with chronic medical conditions (Table 3).
Correlates of Protection after Vaccination
Immune correlates of protection against influenza infection after vaccination include serum hemagglutination
inhibition antibody and neutralization antibody
(15,92). Increased levels of antibody induced by vaccination decrease the risk for
illness caused by strains that are antigenically similar to those strains of the same type or subtype included in the vaccine
(93--96). Although high titers of these antibodies correlate with protection from clinical infection, certain vaccinated persons with
low levels of antibody after vaccination also are protected. The majority of healthy children and adults have high titers of
antibody after vaccination (94,97). However, in certain studies, antibody levels in certain participants declined below levels
considered protective during the year after vaccination, even when the current influenza vaccine contained one or more
antigens administered in previous years
(98,99). Other immunologic correlates of protection that might best indicate
clinical protection after receipt of an intranasal vaccine such as LAIV (e.g., mucosal antibody) are more difficult to measure
(91,100).
Immunogenicity, Efficacy, and Effectiveness of TIV
Children
Children aged >6 months typically have protective levels of anti-influenza antibody against specific influenza virus
strains after influenza vaccination
(92,97,101--106). Children aged 6 months--8 years who have never been vaccinated
previously require 2 doses of TIV separated in time by >4 weeks to induce an optimal serum antibody response.
A study assessing protective antibody responses after 1 and 2 doses of vaccine among children aged 5--8 years who never were
vaccinated previously indicated that children who received
2 doses were substantially more likely than those who received 1 dose to have
a protective antibody response (107). The proportion that had a protective antibody response against the H1N1 antigen and
the H3N2 antigen increased from 67% and 92%, respectively, after the first dose to 93% and 97%, respectively, after the
second dose. However, 36% of children who received
2 doses did not have a protective antibody response
to the influenza B antigen (107).
When the vaccine antigens do not change from one season to the next, priming young children with a single dose of
vaccine in the spring followed by a second dose in the fall engenders similar antibody responses compared with a regimen of 2 doses
in the fall (108). In consecutive years, when vaccine antigens do change, young children who received only 1 dose of vaccine
in their first year of vaccination are less likely to have protective antibody responses when administered only a single dose
during their second year of vaccination, compared with children who received 2 doses in their first year of vaccination
(109,110). An open-label, nonrandomized study compared children aged 6--23 months who received 1 dose of vaccine during the
2003--04 influenza season and a second dose of a different vaccine during the 2004--05 season with children who received 2 doses of
the same vaccine during the 2004--05 season. The proportion that had protective antibody levels against the H3N2
antigen (changed in the second year) or the H1N1 antigen (unchanged) was similar. However, 27% of children who had received
only 1 dose of influenza vaccine during 2003--2004 had a protective antibody response to a single dose of the 2004--2005
vaccine influenza B virus antigen (changed from the previous year), compared with 86% of children who received 2 doses of the
2004--2005 vaccine in their first year of vaccination
(110).
The antibody response among children at high risk for influenza-related complications might be lower than those
typically reported among healthy children
(111,112). However, antibody responses among children with asthma are similar to those
of healthy children and are not substantially altered during asthma exacerbations requiring prednisone treatment
(113).
Multiple studies have demonstrated vaccine efficacy among children aged
>6 months, although efficacy estimates
have varied. In a randomized trial conducted during five influenza seasons (1985--1990) in the United States among children
aged 1--15 years, annual vaccination reduced laboratory-confirmed influenza A substantially (77%--91%)
(94). A limited 1-year placebo-controlled study reported vaccine efficacy of 56% among healthy children aged 3--9 years and 100% among
healthy children and adolescents aged 10--18 years
(114). A retrospective study conducted among approximately 30,000 children
aged 6 months--8 years during an influenza season (2003--04) with a suboptimal vaccine match indicated vaccine effectiveness
of 51% against medically attended, clinically diagnosed pneumonia or influenza (i.e., no laboratory confirmation of
influenza) among fully vaccinated children, and 49% among approximately 5,000 children aged 6--23 months
(115). Another
retrospective study of similar size conducted during the same influenza season in Denver but limited to healthy children
aged 6--21 months estimated clinical effectiveness of 2 TIV doses to be 87% against pneumonia or influenza-related office
visits (116). Among children, TIV efficacy might increase with age
(94,117).
In a nonrandomized controlled trial among children aged 2--6 years and 7--14 years who had asthma, vaccine efficacy
was 54% and 78% against laboratory-confirmed influenza type A infection and 22% and 60% against
laboratory-confirmed influenza type B infection, respectively. Vaccinated children aged 2--6 years with asthma did not have substantially fewer
type B influenza virus infections compared with the control group in this study
(118). Vaccination also might provide
protection against asthma exacerbations
(119); however, other studies of children with asthma have not
demonstrated decreased exacerbations (120). Because of the recognized
influenza-related disease burden among children with other
chronic diseases or immunosuppression and the
long-standing recommendation for vaccination of these children,
randomized placebo-controlled studies to study efficacy in these children have not been conducted because of ethical
considerations.
TIV has been demonstrated to reduce acute otitis media. Two studies have reported that TIV decreases
influenza-associated otitis media approximately 30% among children with mean ages of 20 and 27 months, respectively
(121,122). However, a large study conducted among children with a mean age of 14 months did not provide evidence of TIV
efficacy against acute otitis media (123), although efficacy was 66% against culture-confirmed influenza illness. Influenza
vaccine efficacy against acute otitis media, which is caused by a variety of pathogens and is not typically diagnosed using
influenza virus culture, would be expected to be relatively low because of the nonspecificity of the clinical outcome.
Vaccine Effectiveness for Children Aged 6
Months--8 Years Receiving Influenza Vaccine for
the First Time
Among children aged <8 years who have never received
influenza vaccine previously and who received only 1 dose
of influenza vaccine in their first year of vaccination, vaccine
effectiveness is lower compared with children who receive
2 doses in their first year of being vaccinated. Two recent, large retrospective studies of young children who had received only 1 dose
of TIV in their first year of being vaccinated determined
that no decrease was observed in ILI-related office
visits compared with unvaccinated children
(115,116). Similar results were reported in a case-control study of children aged 6--59
months (124).
When the vaccine antigens do not change from one season to the next, priming with a single dose of vaccine in the
spring followed by a dose in the fall provides a degree of protection against ILI but with substantially lower efficacy compared with
a regimen that provides 2 doses in the fall.
One study conducted over two consecutive seasons in which the vaccine
antigens did not change estimated 62% effectiveness against ILI for healthy children who had received 1 dose in the spring and a second the
following fall, compared with 82% for those who received 2 doses separated by
>4 weeks, both in the fall (116).
Adults Aged <65 Years
TIV is highly immunogenic in healthy adults aged <65 years. Limited or no increase in antibody response is reported
among adults when a second dose is administered during the same season
(125--129). When the vaccine and circulating viruses
are antigenically similar, TIV prevents laboratory-confirmed influenza illness among
approximately 70%--90% of healthy adults aged <65 years in randomized controlled trials
(129--132). Vaccination of healthy adults also has resulted in
decreased work absenteeism and decreased use of health-care resources, including use of antibiotics, when the vaccine and circulating
viruses are well-matched (129--131,133--135).
Efficacy against laboratory-confirmed influenza illness was 50%--77% in
studies conducted during different influenza seasons when the vaccine strains were antigenically dissimilar to the majority
of circulating strains (129,131,135--137). However, protection among healthy adults against
influenza-related hospitalization, measured in the most recent of these studies, was 90%
(137).
In certain studies, persons with certain chronic diseases have lower serum antibody responses after vaccination
compared with healthy young adults and thus can remain susceptible to influenza virus infection and influenza-related upper
respiratory tract illness (138--140). Vaccine efficacy among adults aged <65 years who are at risk for influenza complications is
typically lower than that reported for healthy adults. In a case-control study conducted during 2003--2004, when the vaccine was
a suboptimal antigenic match to many circulating virus strains, effectiveness for prevention of
laboratory-confirmed influenza illness among adults aged 50--64 years with high risk conditions was 48%, compared with 60% for healthy adults
(137). Effectiveness against hospitalization among adults aged 50--64 years with high-risk conditions was 36%, compared with
90% efficacy among healthy adults in that age range
(137).
Studies using less specific outcomes, without laboratory confirmation of influenza virus infection, typically
have demonstrated substantial reductions in hospitalizations or deaths among adults with risk factors for influenza
complications. In a case-control study conducted in Denmark during 1999--2000, vaccination reduced deaths attributable to any cause
78% and reduced hospitalizations attributable to respiratory infections or cardiopulmonary diseases 87%
(141). Benefit was reported after the first vaccination and increased with subsequent vaccinations in subsequent years
(142). Among patients with diabetes mellitus, vaccination was associated with a 56% reduction in any complication, a 54% reduction in
hospitalizations, and a 58% reduction in deaths
(143). Certain experts have noted that the substantial effects on morbidity and
mortality among those who received influenza vaccination in these observational studies should be interpreted with caution because
of the difficulties in ensuring that those who received vaccination had similar baseline health status as those who did not
(90). One meta-analysis of published studies did not determine sufficient evidence to conclude that persons with asthma benefit
from vaccination (144). However, a meta-analysis
that examined efficacy among persons with chronic obstructive pulmonary
disease identified evidence of benefit from vaccination
(145).
TIV produces adequate antibody concentrations against influenza among vaccinated HIV-infected persons who have
minimal AIDS-related symptoms and high CD4+
T-lymphocyte cell counts (146--148). Among persons who have advanced
HIV disease and low CD4+ T-lymphocyte cell counts, TIV might not induce protective antibody titers
(148,149); a second dose of vaccine does not improve the immune response in these persons
(149,150). A randomized, placebo-controlled
trial determined that TIV was highly effective in preventing
symptomatic, laboratory-confirmed influenza virus infection
among HIV-infected persons with a mean of 400 CD4+
T-lymphocyte cells/mm3; however, only a limited number of persons
with CD4+ T-lymphocyte cell counts of <200 were
included in that study (150). A nonrandomized study of HIV-infected
persons determined that influenza vaccination was most effective among persons with >100 CD4+ cells and among those with
<30,000 viral copies of HIV type-1/mL
(70).
Pregnant women have protective concentrations of
anti-influenza antibodies after vaccination
(151,152). Passive transfer of anti-influenza antibodies that might provide protection from vaccinated women to neonates has been reported
(151,153--155). A retrospective, clinic-based study conducted during 1998--2003 reported a nonsignificant trend towards fewer
episodes of MAARI during one influenza season among vaccinated women compared with unvaccinated women and
substantially fewer episodes of MAARI during the peak influenza season
(152). However, a retrospective study conducted during
1997--2002 that used clinical records data did not
observe a reduction in ILI among vaccinated pregnant women or their
infants (156). In another study conducted during 1995--2001, medical visits for respiratory illness among the infants were
not substantially reduced (157). However, studies of influenza vaccine efficacy among pregnant women have not included
specific outcomes such as laboratory-confirmed influenza.
Older Adults
Lower postvaccination anti-influenza antibody concentrations have been reported among certain older persons
compared with younger adults (139--140). A randomized trial among noninstitutionalized persons aged
>60 years reported a vaccine efficacy of 58% against influenza respiratory illness but indicated that efficacy might be lower among those aged
>70 years (158). Among elderly persons not living in nursing homes or similar chronic-care facilities, influenza vaccine is
30%--70% effective in preventing hospitalization for pneumonia and influenza
(159,160). Influenza vaccination reduces the frequency
of secondary complications and reduces the risk for influenza-related hospitalization and death among adults aged
>65 years with and without high-risk medical conditions (e.g., heart disease and diabetes)
(160--165). Influenza vaccine effectiveness
in preventing MAARI among the elderly in nursing homes has been estimated at 20%--40%, but vaccination can be as much
as 80% effective in preventing influenza-related death
(165--168).
Elderly persons typically have a diminished immune response to influenza vaccination compared with young healthy
adults, suggesting that immunity might be of shorter duration and less likely to extend to a second season
(169). Infections among the vaccinated elderly might be related to an age-related reduction in ability to respond to vaccination rather than reduced duration
of immunity.
TIV Dosage, Administration, and Storage
The composition of TIV varies according to manufacturer, and package inserts should be consulted. TIV formulations
in multidose vials typically contain the vaccine preservative thimerosal; preservative-free single dose preparations also
are
available. TIV should be stored at 35°F--46°F (2°C--8°C) and should not be frozen. TIV that has been frozen should
be discarded. Dosage recommendations and schedules vary
according to age group (Table 4). Vaccine prepared for a
previous influenza season should not be administered to provide protection for any subsequent season.
The intramuscular route is recommended for TIV. Adults and older children should be vaccinated in the deltoid muscle.
A needle length of >1 inch (>25 mm) should be considered for persons in these age groups because needles of <1 inch might
be of insufficient length to penetrate muscle tissue in certain adults and older children
(170). When injecting into the deltoid muscle among children with adequate deltoid muscle mass, a needle length of
7/8--1.25 inches is recommended
(171).
Infants and young children should be vaccinated in the
anterolateral aspect of the thigh. A needle length of 7/8--1 inch should be used for children aged <12 months for intramuscular vaccination into the anterolateral thigh.
Adverse Events after Receipt of TIV
In placebo-controlled studies among adults, the most frequent side effect of vaccination was soreness at the vaccination
site (affecting 10%--64% of patients) that lasted <2 days
(130,172,173). These local reactions typically were mild and
rarely interfered with the person's ability to conduct usual daily activities. One study
(112) reported that 20%--28% of children
with asthma aged 9 months--18 years had local pain and swelling at the site of influenza vaccination, and another study
(103) reported that 23% of children aged 6 months--4 years with chronic heart or lung disease had local reactions.
A blinded, randomized, cross-over study of 1,952 adults and children with asthma demonstrated that only self-reported "body
aches" were reported more frequently after TIV (25.1%) than placebo-injection (20.8%)
(174). However, a placebo-controlled
trial of TIV indicated no difference in local reactions among 53 children aged 6 months--6 years with high-risk medical
conditions or among 305 healthy children aged 3--12 years
(104). A recent retrospective study using medical records data
from approximately 45,000 children aged 6--23 months
provided evidence supporting overall safety of TIV in this age
group. Vaccination was not associated with statistically significant increases in any medically attended outcome, and 13
diagnoses, including acute upper respiratory illness, otitis media and asthma, were significantly less common
(175).
Fever, malaise, myalgia, and other systemic symptoms can occur after vaccination with inactivated vaccine and
most often affect persons who have had no previous exposure to the influenza virus antigens in the vaccine (e.g., young
children) (176,177). These reactions begin 6--12 hours after vaccination and can persist for 1--2 days. Recent placebo-controlled
trials demonstrate that among older persons and healthy young adults, administration of split-virus influenza vaccine is
not associated with higher rates of systemic symptoms (e.g., fever, malaise, myalgia, and headache) when compared with
placebo injections (129,172,173,178).
In a randomized cross-over study of children and adults with asthma, no increase in asthma exacerbations was reported
for either age group (174). An analysis of 215,600 children aged <18 years and 8,476 children aged 6--23 months enrolled in
one of five health maintenance organizations (HMOs)
during 1993--1999 reported no increase in biologically plausible,
medically attended events during the 2 weeks after inactivated influenza vaccination, compared with control periods 3--4 weeks
before and after vaccination (179). In a study of 791 healthy children aged 1--15 years
(94), postvaccination fever was noted among 11.5% of those aged 1--5 years, 4.6% among those aged 6--10 years, and 5.1% among those aged 11--15 years.
Among children with high-risk medical conditions, one study of 52 children aged 6 months--3 years reported fever
among 27% and irritability and insomnia among 25%
(103); and a study among 33 children aged 6--18 months reported that
one child had irritability and one had a fever and seizure after vaccination
(180). No placebo comparison group was used in
these studies.
Data regarding potential adverse events after influenza vaccination are available from the Vaccine Adverse Event
Reporting System (VAERS). During January 1991--June 2006, of 25,805 reports of adverse events received by VAERS, 5,727
(22%) concerned children aged <18 years, including 1,070 (4%) children aged 6--23 months (CDC, unpublished data, 2005).
The number of influenza vaccine doses received by children during this entire period is unknown. A recently published review
of VAERS reports submitted after administration of TIV to children aged 6--23 months documented that the most
frequently reported adverse events were fever, rash, injection-site reactions, and seizures; the majority of the limited number of
reported seizures appeared to be febrile
(181). Because of the limitations of passive reporting systems, determining causality for
specific types of adverse events, with the exception of injection-site reactions, usually is not possible using VAERS data
alone. However, a population-based study of TIV safety in children aged 6--23 months who were vaccinated during
1993--1999 identified no adverse events that had a plausible relationship to vaccination
(182).
Immediate and presumably allergic reactions (e.g., hives, angioedema, allergic asthma, and systemic
anaphylaxis) occur rarely after influenza vaccination
(183,184). These reactions probably result from hypersensitivity to certain
vaccine components; the majority of reactions probably are caused by residual egg protein. Although current influenza
vaccines contain only a limited quantity of egg protein, this protein can induce immediate hypersensitivity reactions among
persons who have severe egg allergy. Manufacturers use a variety of different compounds to inactivate influenza viruses and
add antibiotics to prevent bacterial contamination. Package inserts should be consulted for additional information.
Persons who have had hives or swelling of the lips or tongue, or who have experienced acute respiratory distress or
who collapse after eating eggs should consult a physician for
appropriate evaluation to help determine if vaccine should
be administered. Persons who have documented immunoglobulin E (IgE)-mediated hypersensitivity to eggs, including those
who have had occupational asthma related to egg exposure or other allergic responses to egg protein, also might be at
increased risk for allergic reactions to influenza vaccine, and consultation with a physician before vaccination should be considered
(185--187).
Hypersensitivity reactions to vaccine components can
occur but are rare. Although exposure to vaccines
containing thimerosal can lead to hypersensitivity, the majority of
patients do not have reactions to thimerosal when it is administered
as a component of vaccines, even when patch or intradermal tests for thimerosal indicate hypersensitivity
(188,189). When reported, hypersensitivity to thimerosal typically has consisted of local delayed hypersensitivity reactions
(188).
TIV Safety for Persons with HIV Infection
Data demonstrating safety of TIV for HIV-infected persons are limited, but no evidence exists that vaccination has
a clinically important impact on HIV infection or immunocompetence. One study demonstrated a transient (i.e., 2--4
week) increase in HIV RNA (ribonucleic acid) levels in one HIV-infected person after influenza virus infection
(190). Studies have demonstrated a transient increase in replication of HIV-1 in the plasma or peripheral blood mononuclear cells of
HIV-infected persons after vaccine administration
(148,191). However, more recent and better-designed studies have not documented
a substantial increase in the replication of HIV
(192--195). CD4+ T-lymphocyte cell counts or progression of HIV disease
have not been demonstrated to change substantially after influenza vaccination among HIV-infected persons compared
with unvaccinated HIV-infected persons
(148,196). Limited information is available concerning the effect of antiretroviral
therapy on increases in HIV RNA levels after either natural influenza virus infection or influenza vaccination
(64,197).
Guillain-Barré Syndrome and TIV
Guillain-Barré Syndrome (GBS) has an annual incidence of 10--20 cases per 1 million adults
(198). Substantial evidence exists that multiple infectious illnesses, most notably
Campylobacter jejuni gastrointestinal infections and upper respiratory tract infections, are associated with GBS
(199--201). The 1976 swine influenza vaccine was associated with
an increased frequency of GBS (202,203), estimated at one case of GBS per 100,000 persons vaccinated. The risk for
influenza vaccine-associated GBS was higher among persons aged >25 years than among persons aged <25 years
(204). However, obtaining strong epidemiologic evidence for a possible limited increase in risk for a rare condition with multiple causes
is difficult, and evidence for a causal relationship between subsequent vaccines prepared from other influenza viruses and
GBS has not been consistent.
None of the studies conducted using influenza vaccines other than the 1976 swine influenza vaccine have demonstrated
a substantial increase in GBS associated with influenza vaccines. During three of four influenza seasons studied during
1977--1991, the overall relative risk estimates for GBS after influenza vaccination were elevated slightly, but they were not
statistically significant in any of these studies
(205--207). However, in a study of the 1992--93 and 1993--94 seasons, the overall
relative risk for GBS was 1.7 (CI = 1.0--2.8; p = 0.04) during the 6 weeks after vaccination, representing approximately one
additional case of GBS per 1 million persons vaccinated; the combined number of GBS cases peaked 2 weeks after vaccination
(202). Results of a study that examined health-care data from Ontario, Canada, during 1992--2004 demonstrated a small
but statistically significant temporal association
between receiving influenza vaccination and subsequent hospital admission
for GBS. However, no increase in cases of GBS at the population level was reported after introduction of a mass public
influenza vaccination program in Ontario beginning in 2000
(208). Recent data from VAERS have documented decreased reporting
of GBS occurring after vaccination across age groups over time, despite overall
increased reporting of other, non-GBS conditions
occurring after administration of influenza vaccine
(203). Cases of GBS after influenza virus infection have been reported,
but no other epidemiologic studies have documented such an association
(209,210).
If GBS is a side effect of influenza vaccines other than 1976 swine influenza vaccine, the estimated risk for GBS is based
on the few studies that have demonstrated an association between vaccination and GBS is low (i.e., approximately one
additional case per 1 million persons vaccinated). The potential benefits of influenza vaccination in preventing serious
illness, hospitalization, and death substantially outweigh these estimates of risk for vaccine-associated GBS. No evidence indicates
that the case fatality ratio for GBS differs among vaccinated persons and those not vaccinated.
Use of TIV among Patients with a History of GBS
The incidence of GBS among the general population is low, but persons with a history of GBS have a substantially
greater likelihood of subsequently experiencing GBS than persons without such a history
(198). Thus, the likelihood of
coincidentally experiencing GBS after influenza vaccination is
expected to be greater among persons with a history of GBS than
among persons with no history of this syndrome. Whether influenza vaccination specifically might increase the risk for recurrence
of GBS is unknown. However, avoiding vaccinating persons who are not at high risk for severe influenza complications and
who are known to have experienced GBS within 6 weeks after a previous influenza vaccination is prudent. As an
alternative, physicians might consider using influenza antiviral chemoprophylaxis for these persons. Although data are limited,
the established benefits of influenza vaccination might outweigh the risks for many persons who have a history of GBS and
who are also at high risk for severe complications from influenza.
Vaccine Preservative (Thimerosal) in Multidose Vials of TIV
Thimerosal, a mercury-containing anti-bacterial compound, has been used as a preservative in vaccines since the
1930s (211) and is used in multidose vial preparations of TIV to reduce the likelihood of bacterial contamination. No
scientific evidence indicates that thimerosal in vaccines, including influenza vaccines, is a cause of adverse events in vaccine
recipients or to children born to women who received vaccine during pregnancy. In fact, evidence is accumulating that
supports the absence of any risk for neurodevelopment disorders or other harm resulting from exposure to
thimerosal-containing vaccines (212--216). However, continuing public concern about exposure to mercury in vaccines is a potential barrier to achieving
higher vaccine coverage levels and reducing the burden of vaccine-preventable diseases. The U.S. Public Health Service and
other organizations have recommended that efforts be made to eliminate or reduce the thimerosal content in vaccines as part of
a strategy to reduce mercury exposures from all sources
(212,214,216). Since mid-2001, vaccines routinely recommended
for infants aged <6 months in the United States have been manufactured either without or with greatly reduced (trace)
amounts of thimerosal. As a result, a substantial reduction in the total mercury exposure from vaccines for infants and children
already has been achieved (171).
The benefits of influenza vaccination for all recommended groups, including pregnant women and young
children, outweigh the unproven risk from thimerosal exposure through vaccination. The risks for severe illness from influenza
virus infection are elevated among both young children and pregnant women, and vaccination has been demonstrated to
reduce the risk for severe influenza illness and subsequent medical complications. In contrast, no scientifically conclusive evidence
has demonstrated harm from exposure to vaccine containing thimerosal preservative. For these reasons, persons recommended
to receive TIV may receive any age- and risk factor--appropriate vaccine preparation, depending on availability. ACIP and
other federal agencies and professional medical organizations continue to support efforts to provide thimerosal
preservative--free vaccine options.
Nonetheless, certain states have enacted legislation banning the administration of vaccines containing mercury;
the provisions defining mercury content vary
(217). LAIV and many of the single dose vial or syringe preparations of TIV
are thimerosal-free, and the number of influenza vaccine doses that do not contain thimerosal as a preservative is expected
to increase (see Table 4). However, these laws may present a barrier to vaccination unless influenza vaccines that do not
contain thimerosal as a preservative are easily available in those states.
The U.S. vaccine supply for infants and pregnant women
is in a period of transition during which the availability
of thimerosal-reduced or thimerosal-free vaccine intended for these groups is being expanded by manufacturers as a
feasible
means of further reducing an infant's cumulative exposure to mercury. Other environmental sources of mercury exposure
are more difficult or impossible to avoid or eliminate
(212).
LAIV Dosage, Administration, and Storage
Each dose of LAIV contains the same three antigens used in TIV for the influenza season. However, the antigens
are constituted as live, attenuated, cold-adapted,
temperature-sensitive vaccine viruses. Additional components of LAIV
include stabilizing buffers containing monosodium glutamate, hydrolyzed porcine gelatin, arginine, sucrose, and phosphate.
LAIV does not contain thimerosal. LAIV is made from
attenuated viruses and does not cause systemic symptoms of influenza
in vaccine recipients although a minority of recipients experience effects of intranasal vaccine administration or local
viral replication (e.g., nasal congestion)
(218).
In January 2007, a new formulation of LAIV (also sold under the brand name
FluMist™) was licensed that will replace
the older formulation for the 2007--08 influenza season. Compared with the formulation sold previously, the principal
differences are the temperature at which LAIV is shipped and stored after delivery to the clinic and the amount of vaccine
administered. LAIV is intended for intranasal administration only and should not be administered by the intramuscular, intradermal,
or intravenous route. LAIV is not approved for vaccination of children aged <5 years or adults aged >49 years. The
new formulation of LAIV is supplied in a prefilled, single-use sprayer containing 0.2 mL of vaccine. Approximately 0.1 mL
(i.e., half of the total sprayer contents) is sprayed into the first nostril while the recipient is in the upright position. An
attached dose-divider clip is removed from the sprayer to administer the second half of the dose into the other nostril. The
new formulation of LAIV is shipped to end users at 35°F--46°F (2°C--8°C). LAIV should be stored at 35°F--46°F
(2°C--8°C) upon receipt, and can remain at that temperature until the expiration date is reached
(218).
Shedding, Transmission, and Stability of Vaccine
Viruses
Available data indicate that both children and adults vaccinated with LAIV can shed vaccine viruses after
vaccination, although in lower amounts than occur typically with shedding of wild-type influenza viruses. In rare instances, shed
vaccine viruses can be transmitted from vaccine recipients to nonvaccinated persons. However, serious illnesses have not been
reported among unvaccinated persons who have been infected inadvertently with vaccine viruses.
One study of children aged 8--36 months in a child care center assessed transmissibility of vaccine viruses from
98 vaccinated to 99 unvaccinated subjects; 80% of vaccine recipients shed one or more virus strains (mean duration: 7.6
days). One influenza type B vaccine strain isolate was recovered from a placebo recipient and was confirmed to be vaccine-type
virus. The type B isolate retained the cold-adapted, temperature-sensitive, attenuated phenotype, and it possessed the same
genetic sequence as a virus shed from a vaccine recipient who was in the same play group. The placebo recipient from whom
the influenza type B vaccine strain was isolated did not experience any serious clinical events. The estimated probability
of acquiring vaccine virus after close contact with a single LAIV recipient in this child care population was 0.6%--2.4%
(219).
One study assessing shedding of vaccine viruses in 20 healthy vaccinated adults aged 18--49 years demonstrated that
the majority of shedding occurred within the first 3 days after vaccination, although one subject was noted to shed virus on day
7 after vaccine receipt. Duration or type of symptoms associated with receipt of LAIV did not correlate with duration
of shedding of vaccine viruses (220). Another study
assessing shedding of vaccine viruses in 14 healthy adults aged 18--49
years indicated that 50% of these adults had viral antigen detected by direct immunofluorescence or rapid antigen tests within
7 days of vaccination. The majority of viral shedding was detected on day 2 or 3
(221). Vaccine strain virus was detected
from nasal secretions in one (2%) of 57 HIV-infected adults who received LAIV, none of 54 HIV-negative participants
(222), and three (13%) of 23 HIV-infected children compared with seven (28%) of 25 children who were not HIV-infected
(223). No participants in these studies shed virus beyond 10 days after receipt of LAIV. The possibility of person-to-person
transmission of vaccine viruses was not assessed in these studies
(220--223).
In clinical trials, viruses shed by vaccine recipients have been phenotypically stable. In one study, nasal and throat
swab specimens were collected from 17 study participants for 2 weeks after vaccine receipt
(224). Virus isolates were analyzed by multiple genetic techniques. All isolates retained the LAIV genotype after replication in the human host, and all retained
the cold-adapted and temperature-sensitive phenotypes. A study conducted in a child care setting demonstrated that
limited genetic change occurred in the LAIV strains following replication in the vaccine recipients
(225).
Immunogenicity, Efficacy, and Effectiveness of LAIV
The immunogenicity of the approved LAIV has been
assessed in multiple studies conducted among children and
adults (94,226--232). LAIV virus strains replicate primarily in nasopharyngeal epithelial cells. The protective mechanisms
induced by vaccination with LAIV are not understood completely but appear to involve both serum and nasal secretory antibodies.
No single laboratory measurement closely correlates with protective immunity induced by LAIV
(227).
Healthy Children
A randomized, double-blind, placebo-controlled trial among 1,602 healthy children aged 15--71 months assessed
the efficacy of LAIV against culture-confirmed influenza during two seasons
(233,234). This trial included a subset of
children aged 60--71 months who received 2 doses in the first season. In season one (1996--97), when vaccine and circulating
virus strains were well-matched, efficacy against culture-confirmed influenza was 94% for participants who received 2 doses
of LAIV separated by >6 weeks, and 89% for those who received 1 dose. In season two, when the A (H3N2) component in
the vaccine was not well-matched with circulating virus strains, efficacy was 86%, for an overall efficacy over two influenza
seasons of 92%. Receipt of LAIV also resulted in 21% fewer febrile illnesses and a significant
decrease in acute otitis media requiring antibiotics
(233,235). Another randomized, placebo-controlled trial demonstrated 85%--89% efficacy against
culture-confirmed influenza among children aged 6--35 months attending child care centers during consecutive influenza
seasons (236). In one community-based, nonrandomized open-label study, reductions in MAARI were observed among children
who received 1 dose of LAIV during the 1990--00 and 2000--01 influenza seasons even though heterotypic variant influenza
A/H1N1 and B were circulating during that season
(237).
Healthy Adults
A randomized, double-blind, placebo-controlled trial of LAIV effectiveness among 4,561 healthy working adults aged
18--64 years assessed multiple endpoints, including reductions in self-reported respiratory tract illness without
laboratory confirmation, work loss, health-care visits, and medication use during influenza outbreak periods
(238). The study was conducted during the 1997--98 influenza season, when the vaccine and circulating A (H3N2) strains were not
well-matched. The frequency of febrile illnesses was not significantly decreased among LAIV recipients compared with those who
received placebo. However, vaccine recipients had significantly fewer severe febrile illnesses (19% reduction) and
febrile upper respiratory tract illnesses (24% reduction), as well as significant reductions in days of illness, days of work lost, days
with health-care--provider visits, and use of prescription antibiotics and over-the-counter medications
(238). Efficacy against laboratory-confirmed influenza in a randomized, placebo-controlled study was 49%, although efficacy in this study was
not demonstrated to be significantly greater than placebo
(135).
Adverse Events after Receipt of LAIV
Children
In a subset of healthy children aged 60--71 months from one clinical trial
(233), certain signs and symptoms were reported
more often after the first dose among LAIV recipients (n = 214) than among placebo recipients (n =
95), including runny nose (48% and 44%, respectively); headache (18% and 12%, respectively); vomiting (5% and
3%, respectively); and myalgias (6% and 4%, respectively). However, these differences were not statistically significant. In
other trials, signs and symptoms reported after LAIV administration have included runny nose or nasal congestion
(20%--75%), headache (2%--46%), fever (0%--26%), vomiting (3%--13%), abdominal pain (2%), and myalgias
(0%--21%) (94,226,229,231,236,238--241). These symptoms were associated more often with the first dose and were self-limited.
Data from a study including subjects aged 1--17 years indicated an increase in asthma or reactive airways disease among
children aged 18--35 months (241). In another study, medically significant wheezing was more common within 42 days after the
first dose of LAIV (3.2%) compared with TIV (2.0%) among previously unvaccinated children aged 6--23 months,
and hospitalization for any cause within 180 days of vaccination was significantly more common among LAIV (6.1%)
recipients aged 6 months--11 months compared with TIV
recipients (2.6%) (242). Another study was conducted among
>11,000 children aged 18 months--18 years in which 18,780 doses of vaccine were administered for 4 years. For children aged
18
months--4 years, no increase was reported in asthma visits 0--15 days after vaccination compared with the
prevaccination period. A significant increase in asthma events was reported 15--42 days after vaccination, but only in vaccine year 1
(243).
Adults
Among adults, runny nose or nasal congestion (28%--78%), headache (16%--44%), and sore throat (15%--27%) have
been reported more often among vaccine recipients than placebo recipients
(218,244). In one clinical trial among a subset
of healthy adults aged 18--49 years, signs and symptoms reported more frequently among LAIV recipients (n = 2,548)
than placebo recipients (n = 1,290) within 7 days after each dose
included cough (14% and 11%, respectively); runny nose
(45% and 27%, respectively); sore throat (28% and 17%,
respectively); chills (9% and 6%, respectively); and
tiredness/weakness (26% and 22%, respectively)
(244).
Persons at Higher Risk from Influenza-Related Complications
LAIV is currently licensed for use only among healthy nonpregnant persons aged 5--49 years. However, data assessing
the safety of LAIV use for certain groups at risk for influenza-related complications are available. Studies conducted
among children aged 6--71 months with a history of recurrent respiratory infections and among children aged
6--17 years with asthma have not demonstrated differences in postvaccination wheezing or asthma exacerbations, respectively
(245,246). In one study of 54 HIV-infected persons aged
18--58 years and with CD4 counts >200
cells/mm3 who received LAIV, no serious adverse events were reported during a 1-month
follow-up period (222). Similarly, one study demonstrated no
significant difference in the frequency of adverse events or viral shedding among HIV-infected children aged 1--8 years on
effective antiretroviral therapy who were administered LAIV, compared with HIV-uninfected children receiving LAIV
(223). LAIV was well-tolerated among adults aged
>65 years with chronic medical conditions
(247). These findings suggest that persons at
risk for influenza complications who have inadvertent exposure to LAIV would not have significant
adverse events or prolonged viral shedding and that persons who have contact with persons at higher risk for influenza-related complications may
receive LAIV.
Serious Adverse Events
Serious adverse events requiring medical attention among healthy children aged 5--17 years or healthy adults
aged 18--49 years occurred at a rate of <1%
(218). Surveillance will continue for adverse events, including those that might
not have been detected in previous studies. Reviews of reports to VAERS after vaccination of approximately 2.5 million
persons during the 2003--04 and 2004--05 influenza seasons did not indicate any new safety concerns
(248). Health-care professionals should report all clinically significant adverse events promptly to VAERS after LAIV administration.
Comparisons of LAIV and TIV Efficacy
Both TIV and LAIV have been demonstrated to be effective in children and adults, but data directly comparing the
efficacy or effectiveness of these two types of influenza vaccines are limited. Studies comparing the efficacy of TIV to that of
LAIV have been conducted in a variety of settings and populations using several different clinical endpoints. One
randomized, double-blind, placebo-controlled challenge study among 92 healthy adults aged 18--41 years assessed the efficacy of
both LAIV and TIV in preventing influenza infection when challenged with wild-type strains that were antigenically similar
to vaccine strains (249). The overall efficacy in preventing laboratory-documented influenza from all three influenza
strains combined was 85% and 71%, respectively, when challenged 28 days after vaccination by viruses to which study
participants were susceptible before vaccination. The difference in efficacy between the two vaccines was not statistically significant in
this limited study, but efficacy at timepoints later than 28 days after vaccination was not determined. In a randomized,
double-blind, placebo-controlled trial, conducted among young adults during an influenza season when the majority of
circulating H3N2 viruses were antigenically drifted from that season's vaccine viruses, the efficacy of LAIV and TIV against
culture-confirmed influenza was 57% and 77%,
respectively. The difference in efficacy was not statistically significant and was
based largely upon a difference in efficacy against influenza B
(135).
Although LAIV is not currently licensed for use in children aged <5 years or in persons with risk factors for
influenza complications, several studies have compared the efficacy of LAIV to TIV in these groups. LAIV provided 32%
increased protection in preventing culture-confirmed influenza compared with TIV in one study conducted among children
aged
>6 years and adolescents with asthma
(245) and 52% increased protection among children aged 6--71 months with
recurrent respiratory tract infections (245). Another study conducted among children aged 6--71 months during
2004--2005 demonstrated a 55% reduction in cases of
culture-confirmed influenza among children who received LAIV compared
with those who received TIV (242).
Effectiveness of Vaccination for Decreasing Transmission to Contacts
Decreasing transmission of influenza from caregivers and household contacts to persons at high risk might reduce
influenza-related deaths among persons at high risk.
Influenza virus infection and ILI are common among HCP
(250--252). Influenza outbreaks have been attributed to low vaccination rates among HCP in hospitals and long-term--care facilities
(253--255). One serosurvey demonstrated that 23% of HCP had serologic evidence of influenza virus infection during a single
influenza season; the majority had mild illness or subclinical infection
(250). Observational studies have demonstrated that
vaccination of HCP is associated with decreased deaths among nursing home patients
(256,257). In one randomized controlled trial
that included 2,604 residents of 44 nursing homes, significant decreases were determined in mortality, ILI, and medical visits
for ILI care among residents in nursing homes in which staff were offered influenza vaccination (coverage rate: 48%),
compared with nursing homes in which staff were not provided with vaccination (coverage rate: 6%)
(258). A recent review concluded that vaccination of HCP in settings in which patients were also vaccinated provided significant reductions in deaths
among elderly patients from all causes and deaths from pneumonia
(259).
Results from several recent studies have indicated that the benefits of vaccinating children might extend to protection
of their adult contacts and to persons at risk for influenza complications in the community, including persons at risk
for influenza complications. A single-blinded, randomized controlled study conducted during 1996--1997 trial demonstrated
that vaccinating preschool-aged children with TIV reduced
influenza-related morbidity among their household contacts
(260). A community-based observational study conducted during the 1968 pandemic using a univalent inactivated
vaccine reported that a vaccination program targeting school-aged children (coverage rate: 86%) in one community reduced
influenza rates within the community among all age groups compared with another community in which aggressive vaccination was
not conducted among school-aged children
(261). An observational study conducted in Russia demonstrated reductions in
ILI among the community-dwelling elderly after implementation of a vaccination program using TIV for children aged 3--6
years (57% coverage achieved) and children and adolescents aged 7--17 years (72% coverage achieved)
(262). A randomized, placebo-controlled trial among children with recurrent respiratory tract infections demonstrated that members of families
with children who had received LAIV were significantly less likely to have respiratory tract infections and reported
significantly fewer workdays lost, compared with families with children who received placebo
(263). In nonrandomized community-based studies, administration of LAIV has been demonstrated to reduce MAARI
(264,265) and ILI-related economic and
medical consequences (e.g., workdays lost and number of health-care provider visits) among contacts of vaccine recipients
(265). Households with children attending schools in which school-based LAIV
immunization programs had been established reported less ILI and fewer physician visits during peak influenza season, compared with households with children in
schools in which no LAIV immunization had been offered. However a decrease in the overall rate of school absenteeism was
not reported in communities in which LAIV immunization was offered
(265).
Cost-Effectiveness of Influenza Vaccination
Influenza vaccination can reduce both health-care costs and productivity losses associated with influenza illness. Studies
of influenza vaccination of persons aged
>65 years conducted in the United States have reported substantial reductions
in hospitalizations and deaths and overall societal cost savings
(159,160,266). Studies of adults aged <65 years have reported
that vaccination can reduce both direct medical costs and
indirect costs from work absenteeism
(129,130,132--134,267). Influenza vaccination has been estimated to decrease costs
associated with influenza illness, including 13%--44% reductions in
health-care--provider visits, 18%--45% reductions in lost workdays, 18%--28% reductions in days working with
reduced effectiveness, and 25% reductions in antibiotic use for influenza-associated illnesses
(129,131,268,269). One analysis estimated a cost of approximately $4,500 per illness averted among healthy persons aged 18--64 years in a typical season,
with cost/case averted decreasing to as low as $60 when the influenza attack rate and vaccine effectiveness against ILI are
high
(130). Another cost-benefit analysis that also included costs from lost work productivity estimated an average
annual savings of $13.66 per person vaccinated
(270).
Economic studies specifically evaluating the
cost-effectiveness of vaccinating persons in other age groups
currently recommended for vaccination (e.g., persons aged 50--64 years or children aged 6--59 months) are limited and
typically demonstrate much higher costs in these healthier populations
(266,271--274). In a study of inactivated vaccine that
included persons in all age groups, cost utility (i.e., cost per year of healthy life gained) improved with increasing age and among
those with chronic medical conditions
(266). Among persons aged >65 years, vaccination resulted in a net savings per
quality-adjusted life year (QALY) saved. Another study estimated the cost-effectiveness of influenza vaccination to be $28,000
per QALY saved (in 2000 dollars) in persons aged 50--64 years compared with $980 per QALY saved among persons aged
>65 years (275).
Cost analyses have documented the considerable cost burden of illness among children. In a study of 727 children at a
single medical center during 2000--2004, the mean total cost of hospitalization for influenza-related illness was $13,159
($39,792 for patients admitted to an intensive care unit and $7,030 for patients cared for exclusively on the wards)
(276). Strategies that focus on vaccinating children with medical conditions that confer a higher risk for influenza complications appear to be
more cost-effective than a strategy of vaccinating all children
(277). An analysis that compared the costs of vaccinating children
of varying ages with TIV and LAIV determined that costs per QALY saved increased with age for both vaccines. In 2003
dollars per QALY saved, costs for routine vaccination using TIV were $12,000 for healthy children aged 6--23 months and
$119,000 for healthy adolescents aged 12--17 years, compared with $9,000 and $109,000 using LAIV, respectively
(278).
Vaccination Coverage Levels
Continued annual monitoring is needed to determine the effects on vaccination coverage of vaccine supply delays
and shortages, changes in influenza vaccination recommendations and target groups for vaccination, reimbursement rates
for vaccine and vaccine administration, and other factors related to vaccination coverage among adults and children.
National health objectives for 2010 include achieving an influenza vaccination coverage level of 90% for persons aged
>65 years and among nursing home residents
(279,280), but new strategies to improve coverage are needed to achieve these objectives
(281--282). Increasing vaccination coverage among persons who have high-risk conditions and are aged <65 years,
including children at high risk, is the highest priority for expanding influenza vaccine use.
On the basis of preliminary data from the National Health Interview Survey (NHIS), estimated national influenza
vaccine coverage in the second quarter of 2006 among persons aged >65 years and 50--64 years was 66% and 32%, respectively
(283). Compared with coverage estimates from the 2005 NHIS, coverage in these age groups
has increased (Table 5) (283). In early October 2004, one of the influenza vaccine manufacturers licensed in the United States announced that it would be unable
to supply any vaccine to the United States, causing an abrupt and substantial decline in vaccine availability and prompting
ACIP to recommend that vaccination efforts target certain groups at higher risk for influenza complications. The inability of
this manufacturer to produce vaccine for the United States reduced by almost one half the expected supply of TIV available for
the 2004--05 influenza season (284,285). Although vaccine supply was adequate for the 2005--06 influenza season, recent
trends in vaccination coverage are difficult to
interpret until analyses of recent NHIS vaccination coverage data are completed.
During 1989--1999, influenza vaccination levels among persons aged
>65 years increased from 33% to 66%
(286,287), surpassing the Healthy People
2000 objective of 60% (281). Possible reasons for increases in influenza vaccination levels
among persons aged >65 years include 1) greater acceptance of preventive medical services by practitioners; 2) increased delivery
and administration of vaccine by health-care providers and sources other than physicians; 3) new information regarding
influenza vaccine effectiveness, cost-effectiveness, and safety; and 4) initiation of Medicare reimbursement for influenza vaccination
in 1993 (129,160,166,167,288,289). However, since 1997, increases in influenza vaccination coverage levels among the
elderly have slowed markedly, with coverage estimates during years without vaccine shortages since 1997 ranging between 63%
and 66%.
In 2004, estimated vaccination coverage levels among adults with high-risk conditions aged 18--49 years and 50--64
years were 26% and 46%, respectively, substantially lower than the
Healthy People 2000 and Healthy People
2010 objectives of 60% (Table 5)
(279,280). In 2005, vaccination coverage among persons in these groups decreased to 18% and 34%,
respectively; vaccine shortages during the previous influenza season likely contributed to these declines in coverage.
Opportunities to vaccinate persons at risk for influenza complications (e.g., during hospitalizations for other
causes) often are missed. In a study of hospitalized Medicare patients, only 31.6% were vaccinated before admission, 1.9%
during admission, and 10.6% after admission
(290). A study conducted in New York City during 2001--2005 among 7,063
children aged 6--23 months determined that 2-dose vaccine coverage increased from 1.6% to 23.7%. Although the average number
of medical visits during which an opportunity to be vaccinated decreased during the course of the study from 2.9 to 2.0
per child, 55% of all visits during the final year of the study still represented a missed vaccination opportunity
(291). Using standing orders in hospitals increases vaccination rates among hospitalized persons
(292). In one survey, the strongest predictor of receiving vaccination was the survey respondent's belief that he or she was in a high-risk group. However,
many persons in high-risk groups did not know that they were in a group recommended for vaccination
(293).
Reducing racial and ethnic health disparities, including disparities in influenza vaccination coverage, is an
overarching national goal that is not being met
(279). Although estimated influenza vaccination coverage for the 1999--00 season
reached the highest levels recorded among older black, Hispanic, and white populations, vaccination levels among blacks
and Hispanics continue to lag behind those among whites
(287,294). Estimated vaccination coverage levels in 2005
among persons aged >65 years were 68% for non-Hispanic whites, 47% for non-Hispanic blacks, and 49% for Hispanics
(283). Among Medicare beneficiaries, unequal access to care might not be the only factor in contributing toward disparity
levels; other key factors include having patients that actively seek vaccination and providers that recommend vaccination
(295,296). One study estimated that eliminating these disparities in vaccination coverage would have an impact on mortality similar
to the impact of eliminating deaths attributable to kidney disease among blacks or liver disease among Hispanics
(297).
Reported vaccination levels are low among children at
increased risk for influenza complications. Coverage among
children aged 2--17 years with asthma for the 2004--05 influenza season was estimated to be 29%
(298). One study reported 79% vaccination coverage among children attending a cystic fibrosis treatment center
(299). During the first season for which ACIP recommended that all children aged 6 months--23 months receive vaccination, 33% received
>1 dose of influenza vaccination, and 18% received 2 doses if they were unvaccinated previously
(300). Among children enrolled in HMOs who had received a first dose during 2001--2004, second dose coverage varied from 29% to 44% among children aged
6--23 months and from 12% to 24% among children aged 2--8 years
(301). A rapid analysis of influenza vaccination coverage
levels among members of an HMO in Northern California demonstrated that during 2004--2005, the first year of
the recommendation for vaccination of children aged 6--23 months, 1-dose coverage was 57%
(302). Data collected in February 2005 indicated a national estimate of 48% vaccination coverage for
>1 doses among children aged 6--23 months and
35% coverage among children aged 2--17 years who had one or more high-risk medical conditions during the 2004--05
season (303). As has been reported for older adults, a physician recommendation for vaccination and the perception that having
a child be vaccinated "is a smart idea" were associated positively with likelihood of vaccination of children aged 6--23
months (304). Similarly, children with asthma were more likely to be vaccinated if their parents
recalled a physician recommendation to be vaccinated or
believed that the vaccine worked well (305). Implementation of a reminder/recall system in a
pediatric clinic increased the percentage of children with asthma or reactive airways disease receiving vaccination from 5% to
32% (306).
Although annual vaccination is recommended for HCP and is a high priority for reducing morbidity associated
with influenza in health-care settings and for expanding influenza vaccine use
(307--309), national survey data demonstrated
a vaccination coverage level of only 42% among HCP (CDC, unpublished data, 2006). Vaccination of HCP has
been associated with reduced work absenteeism
(251) and with fewer deaths among nursing home patients
(257,258) and elderly hospitalized patients
(260). Factors associated with a higher rate of influenza vaccination among HCP include older age,
being a hospital employee, having employer provided health-care insurance, having had pneumococcal or hepatitis B vaccination
in the past, or having visited a health-care professional during the previous year. Non-Hispanic black HCP were less likely
than non-Hispanic white HCP to be vaccinated
(310).
Limited information is available regarding influenza vaccine coverage among pregnant women. In a national
survey conducted during 2001 among women aged 18--44 years without diabetes, those who were pregnant were significantly
less likely to report influenza vaccination during the previous 12 months (13.7%) than those women who were not
pregnant (16.8%) (311). Only 16% of pregnant women participating in the 2005 NHIS reported vaccination, excluding
pregnant women who reported diabetes, heart disease, lung disease, and other selected high-risk conditions (CDC, unpublished
data, 2006) (Table 5). In a study of influenza vaccine acceptance by pregnant women, 71% of those who were offered the
vaccine
chose to be vaccinated (312). However, a 1999 survey of
obstetricians and gynecologists determined that only
39% administered influenza vaccine to obstetric patients in their practices, although 86% agreed that pregnant women's risk
for influenza-related morbidity and mortality increases during the last two trimesters
(313).
Data indicate that self-report of influenza vaccination among adults, compared with determining vaccination status
from the medical record, is both a sensitive and specific source of information
(314). Patient self-reports should be accepted
as evidence of influenza vaccination in clinical practice
(314). However, information on the validity of parents' reports
of pediatric influenza vaccination is not yet available.
Recommendations for Using TIV and LAIV During the 2007--08
Influenza Season
Both TIV and LAIV prepared for the 2007--08 season will include A/Solomon Islands/3/2006 (H1N1)-like,
A/Wisconsin/ 67/2005 (H3N2)-like, and B/Malaysia/2506/2004-like antigens. These viruses will be used because they are representative
of influenza viruses that are anticipated to circulate in the United States during the 2007--08 influenza season and have
favorable growth properties in eggs.
TIV and LAIV can be used to reduce the risk for influenza virus infection and its complications. Immunization
providers should administer influenza vaccine to any person who wishes to reduce the likelihood of becoming ill with influenza
or transmitting influenza to others should they become
infected. Healthy, nonpregnant persons aged 5--49 years can choose
to receive either vaccine.
TIV is FDA-approved for persons aged >6 months, including those with high-risk conditions, whereas LAIV is
FDA-approved for use only among healthy persons aged 5--49 years. All children aged >6 months--8 years who have not
been vaccinated previously at any time with either LAIV or TIV should receive 2 doses of age-appropriate vaccine in the
same season, with a single dose during subsequent seasons.
Target Groups for Vaccination
All persons at risk for medical complications from influenza or more likely to require medical care and all persons who
live with or care for persons at high risk for influenza-related complications should receive influenza vaccine
annually. Approximately 73% of the United States population is
included in one or more of these target groups; however, only
an estimated one third of the United States population
received an influenza vaccination in 2006--2007. When vaccine supply
is limited, vaccination efforts should focus on delivering
vaccination to these persons.
Persons at Risk for Medical Complications or More
Likely to Require Medical Care
Vaccination with TIV is recommended for the following persons who are at increased risk for severe complications
from influenza, or at higher risk for influenza-associated clinic, emergency department, or hospital visits:
- all children aged 6--59 months (i.e., 6 months--4 years;
- all persons aged >50 years;
- children and adolescents (aged 6 months--18 years) who are receiving long-term aspirin therapy and who therefore
might be at risk for experiencing Reye syndrome after
influenza virus infection;
- women who will be pregnant during the influenza season;
- adults and children who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal,
hepatic, hematological or metabolic disorders (including diabetes mellitus);
- adults and children who have immunosuppression
(including immunosuppression caused by medications or by HIV);
- adults and children who have any condition (e.g., cognitive dysfunction, spinal cord injuries, seizure disorders, or
other neuromuscular disorders) that can compromise respiratory function or the handling of respiratory secretions or that
can increase the risk for aspiration; and
- residents of nursing homes and other chronic-care facilities.
Persons Who Live With or Care for Persons at High Risk for Influenza-Related Complications
To prevent transmission to persons identified above, vaccination with TIV or LAIV (unless contraindicated) also
is recommended for the following persons:
- HCP;
- healthy household contacts (including children) and caregivers of children aged
<59 months (i.e., aged <5 years) and adults aged
>50 years; and
- healthy household contacts (including children) and caregivers of persons with medical conditions that put them at
higher risk for severe complications from
influenza.
Additional Information Regarding Vaccination of Specific Populations
Children
Any child aged >6 months may be vaccinated. However, vaccination is specifically recommended for certain
children, including all children aged 6--59 months, children with certain medical conditions, and children who are contacts of
persons at higher risk for influenza complications. The American Academy of Pediatrics (AAP) has developed an algorithm
for determining specific recommendations for pediatric
patients according to age, contact or health status has been
provided (Figure).
Because children aged 6--23 months are at substantially
increased risk for influenza-related hospitalizations, and
children aged 24--59 months (i.e., 2--4 years) are at increased risk for influenza-related clinic and emergency department visits
(34), ACIP recommends that all children aged 6
months--4 years receive TIV. Influenza vaccines are not approved by FDA for
use among children aged <6 months.
All children aged 6 months--8 years who have not received vaccination against influenza previously should receive 2 doses
of vaccine the first year they are vaccinated. Children aged
5--8 years who receive TIV should have a booster dose of
TIV administered >1 month after the initial dose, ifpossible before the onset of influenza season. LAIV is not currently licensed
for children aged <5 years. Children aged 5--8 years who
receive LAIV should have a second dose of LAIV 6 or more weeks
after the initial dose. If possible, both doses should be administered before onset of influenza season. However,
vaccination, including the second dose, is recommended even after influenza virus
begins to circulate in a community.
Although data are limited, recently published studies indicate that when young children receive only 1 dose of TIV in
each of their first two seasons of being vaccinated, they have lower antibody levels, are less likely to have protective antibody
titers (110), and have reduced protection against ILI compared with children who receive their first 2 doses of vaccine in the
same season (116). ACIP recommends 2 vaccine doses for children aged 6 months--8 years who received an influenza
vaccine (either TIV or LAIV) for the first time in the previous season but who did not receive the recommended second dose
of vaccine within the same season. ACIP recommendations are now harmonized with regard to this issue with those of
AAP (315). This recommendation represents a change from the 2006 recommendations, in which children aged
6 months--8 years who received only 1 dose of vaccine in their first year of vaccination were recommended to receive only a single dose in
the following season. ACIP does not recommend that a child receive influenza vaccine for the first time in the spring with
the intent of providing a priming dose for the following season. Children recommended for vaccination who are in their third
or more year of being vaccinated and who received only 1 dose in each of their first
2 years of being vaccinated should continue receiving a single annual dose.
Persons Aged 50--64 Years
Vaccination is recommended for all persons aged 50--64 years because persons in this age group have an
increased prevalence of high-risk conditions and low vaccination rates. In 2002, approximately 43.6 million persons in the United
States were aged 50--64 years, of whom 13.5 million (34%) had one or more high-risk medical conditions
(316). Persons aged 50--64 years without high-risk conditions also benefit from vaccination in the form of decreased rates of
influenza illness, work absenteeism, and need for medical visits and medications, including antibiotics
(128,129,131,132). In addition,
other preventive services and routine assessment of vaccination and other preventive services has been recommended for all
persons aged >50 years (317,318).
HCP and Other Persons Who Can Transmit Influenza to Those at High Risk
Healthy persons who are clinically or asymptomatically
infected can transmit influenza virus to persons at higher risk
for complications from influenza. In addition to HCP, groups that can transmit influenza to high-risk persons and that should
be vaccinated include
- employees of assisted living and other residences for persons in groups at high risk;
- persons who provide home care to persons in groups at high risk; and
- household contacts (including children) of persons in groups at high risk.
In addition, because children aged <5 years are at increased risk for influenza-related hospitalization
(2,33,55,57) compared with older children, vaccination is recommended for their household contacts and out-of-home caregivers. Because
influenza vaccines have not been approved by FDA for use among children aged <6 months, emphasis should be placed on
vaccinating contacts of children aged <6 months. When vaccine supply is limited, priority for vaccination should be given to contacts
of children aged <6 months.
Healthy persons aged 5--49 years in these groups who are not contacts of severely immunosuppressed persons
(see Vaccination of Close Contacts of Immunocompromised Persons) may receive either LAIV or TIV. All other persons
should receive TIV.
All HCP, as well as those in training for health-care professions, should be vaccinated annually against influenza.
Persons working in health-care settings who should be vaccinated include physicians, nurses, and other workers in both hospital
and outpatient-care settings, medical emergency-response workers (e.g., paramedics and emergency medical
technicians), employees of nursing home and chronic-care facilities who have contact with patients or residents, and students in
these professions who will have contact with patients
(308,309,319).
Facilities that employ HCP should provide vaccine to workers by using approaches that have been demonstrated to
be effective in increasing vaccination coverage. Health-care
administrators should consider the level of vaccination
coverage among HCP to be one measure of a patient safety quality program and obtain signed declinations from personnel who
decline influenza vaccination for reasons other than medical contraindications
(309). Influenza vaccination rates among HCP
within facilities should be regularly measured and
reported, and ward-, unit-, and specialty-specific coverage rates should be
provided to staff and administration (309). Studies have demonstrated that organized campaigns can attain higher rates of
vaccination among HCP with moderate effort and using strategies that increase vaccine acceptance
(307,309,320).
Efforts to increase vaccination coverage among HCP are supported by various national accrediting and
professional organizations and in certain states by statute. The Joint Commission on Accreditation of Health-Care Organizations
has approved an infection control standard that requires accredited organizations to offer influenza vaccinations to
staff, including volunteers and licensed independent practitioners with close patient contact. The standard became an
accreditation requirement beginning January 1, 2007
(321). In addition, the Infectious Diseases Society of America recently
recommended mandatory vaccination for HCP, with a provision for declination of vaccination based on religious or medical reasons
(322). Fifteen states have regulations regarding vaccination of HCP in long-term--care facilities
(323), three states require that health-care facilities offer influenza vaccination to HCP, and three states require that HCP either receive
influenza vaccination or indicate a religious, medical, or philosophical reason for not being
vaccinated (324).
Vaccination of Close Contacts of Immunocompromised
Persons
Immunocompromised persons are at risk for influenza complications but might have insufficient responses to
vaccination. Close contacts of immunocompromised persons, including HCP, should be vaccinated to reduce the risk for
influenza transmission. TIV is preferred for vaccinating household members, HCP, and others who have close contact with
severely immunosuppressed persons (e.g., patients with
hematopoietic stem cell transplants) during those periods in which
the immunosuppressed person requires care in a protective environment (typically defined as a specialized patient-care area with
a positive airflow relative to the corridor, high-efficiency particulate air filtration, and frequent air changes)
(325).
LAIV transmission from a recently vaccinated person causing clinically important illness in an
immunocompromised contact has not been reported. The rationale for avoiding use of LAIV among HCP caring for such patients is
the theoretic risk that a live, attenuated vaccine virus could be transmitted to the severely immunosuppressed person. As
a precautionary measure, HCP who receive LAIV should avoid providing care for severely immunosuppressed patients
for
7 days after vaccination. Hospital visitors who have received LAIV should avoid contact with severely
immunosuppressed persons for 7 days after vaccination but should not be restricted from visiting less severely immunosuppressed patients.
No preference is indicated for TIV use by persons who have close contact with persons with lesser degrees
of immunosuppression (e.g., persons with diabetes, persons with asthma who take corticosteroids, those who might have been
cared for previously in a protective environment but who are no longer in that protective environment, or persons infected with
HIV) or for TIV use by HCP or other healthy persons aged 5--49 years in close contact with persons in all other groups at high risk.
Pregnant Women
Pregnant women are at risk for influenza complications, and all women who are pregnant or will be pregnant
during influenza season should be vaccinated. FDA has classified TIV as a "Pregnancy Category C" medication, indicating
that animal reproduction studies have not been conducted. Whether
influenza vaccine can cause fetal harm when administered to
a pregnant woman or affect reproductive capacity is not known. However, one study of approximately 2,000 pregnant
women who received TIV during pregnancy demonstrated no adverse fetal effects and no adverse effects during infancy or
early childhood (326). A matched case-control study of 252 pregnant women who received TIV within the 6 months
before delivery determined no adverse events after vaccination among pregnant women and no difference in pregnancy
outcomes compared with 826 pregnant women who were not vaccinated
(152). During 2000--2003, an estimated 2 million
pregnant women were vaccinated, and only 20 adverse events among women who received TIV were reported to VAERS during
this time, including nine injection-site reactions and eight systemic reactions (e.g., fever, headache, and myalgias). In
addition, three miscarriages were reported, but these were not known to be causally related to vaccination
(327). Similar results have been reported in several smaller studies
(151,153,328) The American College of Obstetricians and Gynecologists and
the American Academy of Family Physicians also have recommended routine vaccination of all pregnant women
(329). No preference is indicated for use of TIV that does not contain thimerosal as a preservative (see Vaccine Preservative
[Thimerosal] in Multidose Vials of TIV) for any group recommended for vaccination, including pregnant women. LAIV is not licensed
for use in pregnant women. However, pregnant women do not need to avoid contact with persons recently vaccinated with LAIV.
Breastfeeding Mothers
Vaccination is recommended for all persons, including breastfeeding women, who are contacts of infants or children
aged <59 months (i.e., <5 years), because infants and young children are at higher risk for influenza complications and are
more likely to require medical care or hospitalization if
infected. Breastfeeding does not affect the immune response adversely and
is not a contraindication for vaccination
(171). Women who are breastfeeding may receive either TIV or LAIV
unless contraindicated because of other medical conditions.
Travelers
The risk for exposure to influenza during travel depends on the time of year and destination. In the temperate regions of
the Southern Hemisphere, influenza activity occurs typically during April--September. In temperate climate zones of the
Northern and Southern Hemispheres, travelers also can be exposed to influenza during the summer, especially when traveling as part
of large tourist groups (e.g., on cruise ships) that include persons from areas of the world in which influenza viruses
are circulating (330,331). In the tropics, influenza occurs throughout the year. In one recent study among Swiss travelers
to tropical and subtropical countries, influenza was the most frequently acquired vaccine-prev |
