Contact Us
- 800–CDC–INFO
- (800-232-4636)
- 888–232–6348 (TTY)
- cdcinfo@cdc.gov
ACIP Recommendations:
Introduction & Biology of Influenza
NOTE: The text below is taken directly from Prevention & Control of Influenza - Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2007 Jul 13;56(RR06):1-54. Also available as PDF (720K).
On this page:
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. 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 and older, children aged <2 years, and persons of any age who have medical conditions that place them at increased risk for complications from influenza.
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. Influenza vaccine may be administered to any person aged 6 months and older 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.
Annual vaccination against influenza is recommended for:
- all persons, including school-aged children, who want to reduce the risk of becoming ill with influenza or of transmitting influenza to others
- all children aged 6–59 months (i.e., 6 months–4 years);
- all persons aged 50 years and older;
- children and adolescents (aged 6 months–18 years) receiving long-term aspirin therapy 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 human immunodeficiency virus;
- 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;
- residents of nursing homes and other chronic-care facilities;
- health-care personnel;
- healthy household contacts (including children) and caregivers of children aged <5 years and adults aged 50 years and older, with particular emphasis on vaccinating contacts of children aged <6 months; and
- healthy household contacts (including children) and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza.
Trivalent inactivated influenza vaccine (TIV) may be used for any person aged 6 months and older, 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.
Biology of Influenza
Influenza A and B are the two types of influenza viruses that cause epidemic human disease. 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. 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. 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. 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.
- Page last updated October 26, 2007
- Content Source: Coordinating Center for Infectious Diseases (CCID)
- National Center for Immunization and Respiratory Diseases (NCIRD)
