Chapter 2: Haemophilus influenzae type b (Hib)
Manual for the Surveillance of Vaccine-Preventable Diseases
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Authors: Elizabeth C. Briere, MD, MPH; Xin Wang, PhD
Haemophilus influenzae (Hi) invasive disease is caused by the bacterium Haemophilus influenzae. H. influenzae may be either encapsulated (typeable) or unencapsulated (nontypeable). Six antigenically distinct capsular types of H. influenzae (types a–f) that can cause invasive disease in persons of any age have been identified. Nontypeable strains can also cause invasive disease but more commonly cause mucosal infections.
Invasive H. influenzae diseases include clinical syndromes of meningitis, bacteremia or sepsis, epiglottitis, pneumonia, septic arthritis, osteomyelitis, pericarditis, and cellulitis. In contrast, syndromes of mucosal infections such as bronchitis, sinusitis, and otitis media are considered noninvasive disease. The noninvasive syndromes are not nationally notifiable.
Before the introduction of effective vaccines, H. influenzae serotype b (Hib) was the cause of more than 95% of invasive H. influenzae diseases among children younger than 5 years of age. Hib was the leading cause of bacterial meningitis in the United States among children younger than 5 years of age and a major cause of other life–threatening invasive bacterial diseases in this age group. Meningitis occurred in approximately two–thirds of children with invasive Hib disease, resulting in hearing impairment or severe permanent neurologic sequelae, such as mental retardation, seizure disorder, cognitive and developmental delay, and paralysis in 15%–30% of survivors. Approximately 4% of all cases were fatal.
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Since the introduction of the Hib polysaccharide and conjugate vaccines in 1985 and 1990, the
incidence of invasive Hib disease in children less than 5 years of age has decreased by 99%, to
less than 1 case per 100,000 children younger than 5 years of age.[2–5] Continued monitoring of
invasive H. influenzae disease through the Active Bacterial Core surveillance (ABCs) system,
which includes serotype information on all invasive H. influenzae isolates, has demonstrated
low rates of invasive Hib in children younger than 5 years of age; between 2010–2013, the
average incidence was 0.14 cases per 100,000, which is below the Healthy People 2020 goal of 0.27/100,000.[6–10]
In the post–Hib vaccine era, the epidemiology of invasive H. influenzae disease in the United
States has changed. Studies suggest that the majority of invasive H. influenzae disease in all age
groups is now caused by nontypable H. influenzae.
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Importance of Rapid Case Identification
Rapid identification of cases is important to allow for early administration of chemoprophylaxis
and Hib vaccine, if needed, to household and childcare classroom contacts of case–patients.
Early notification of H. influenzae invasive disease cases in children younger than 5 years
is also important to ensure isolates are saved for serotyping. State health departments with
questions about serotyping should contact the CDC Meningitis and Vaccine–Preventable
Diseases Branch laboratory at 404-639-3158.
Importance of Surveillance
H. influenzae surveillance information is used to monitor the effectiveness of Hib immunization programs and vaccines, to assess progress toward Hib disease elimination, and to describe the
epidemiology of non-b invasive H. influenzae disease.
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Disease Reduction Goals
Hib disease has declined rapidly because of widespread immunization of infants and young
children with conjugate vaccines and because humans are the only known reservoir for Hib.
The following case definition for invasive H. influenzae disease has been approved by the
Council of State and Territorial Epidemiologists (CSTE) and was published in 2014.
Clinical case description
Invasive disease caused by H. influenzae can produce any of several clinical syndromes, including meningitis, bacteremia, epiglottitis, pneumonia, septic arthritis, cellulitis, or purulent pericarditis; endocarditis and osteomyelitis occur less commonly.
Laboratory criteria for diagnosis
- Detection of Haemophilus influenzae type b antigen in cerebrospinal fluid [CSF]
- Detection of Haemophilus influenzae-specific nucleic acid in a specimen obtained from a normally sterile body site (e.g., blood or CSF), using a validated polymerase chain reaction (PCR) assay
- Isolation of H. influenzae from a normally sterile body site (e.g., CSF, blood, joint fluid, pleural fluid, or pericardial fluid)
Probable: Meningitis with detection of H. influenzae type b antigen in cerebralspinal fluid [CSF]
- Isolation of H. influenzae from a normally sterile body site (e.g., cerebrospinal fluid [CSF], blood, joint fluid, pleural fluid, pericardial fluid)
- Detection of Haemophilus influenzae -specific nucleic acid in a specimen obtained from a normally sterile body site (e.g., cerebrospinal fluid [CSF], blood, joint fluid, pleural fluid, pericardial fluid), using a validated polymerase chain reaction (PCR) assay
Comment: Positive antigen detection test results from urine or serum samples are unreliable for diagnosis of H. influenzae disease.
Isolates of Haemophilus influenzae are important for antimicrobial susceptibility testing.
[The positive antigen test results can occur from circulation of Hib antigen in urine or serum;
this circulation can be caused by asymptomatic Hib carriage, recent vaccination, or fecal contamination of urine specimens. Cases identified exclusively by these methods should be considered suspect cases only.]
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Confirming a case of Hib disease requires culturing and isolating the bacteria from a normally
sterile body site. Most hospital and commercial microbiologic laboratories have the ability to
isolate H. influenzae from cultured specimens. Normally sterile-site specimens for isolation
of invasive H. influenzae include CSF, blood, joint fluid, pleural effusion, pericardial effusion,
peritoneal fluid, subcutaneous tissue fluid, placenta, and amniotic fluid. All H. influenzae
isolates should be tested for antimicrobial susceptibility according to guidelines in M2-A9
Performance Standards for Antimicrobial Disk Susceptibility Tests (January 2006) from the
Clinical Laboratory Standards Institute.
Serotype testing (serotyping)
Serotyping distinguishes encapsulated strains, including Hib, from unencapsulated strains,
which cannot be serotyped. The six encapsulated serotypes (designated a–f) have distinct
capsular polysaccharides that can be differentiated by slide agglutination with type-specific
To make public health decisions about chemoprophylaxis, microbiology laboratories should
perform serotype testing of H. influenzae isolates. Even though Hib disease has declined,
laboratories should continue routine serotyping. If serotyping is not available at a laboratory,
laboratory personnel should contact the state health department. State health departments
with questions about serotyping should contact the CDC Meningitis and Vaccine Preventable
Diseases Branch laboratory at 404-639-3158.
Because the type b capsular antigen can be detected in body fluids, including urine, blood, and
CSF of patients, clinicians often request a rapid antigen detection test for diagnosis of Hib
disease. Antigen detection may be used as an adjunct to culture, articularly in the diagnosis of patients who have received antimicrobial agents before specimens are obtained for culture. The
method for antigen detection is latex agglutination (LA). LA is a rapid and sensitive method
used to detect Hib capsular polysaccharide antigen in CSF, serum, urine, pleural fluid, or joint
fluid but false negative and false positive reactions can occur.
If the Hib antigen is detected in CSF but a positive result is not obtained from culture of sterile
site, the patient should be considered as having a probable case of Hib disease and reported as
such. Because antigen detection tests can be positive in urine and serum of persons without
invasive Hib disease, persons who are identified exclusively by positive antigen tests in urine
or serum should not be reported as cases.
Although culture is the gold standard for confirming Haemophilus influenzae, real-time PCR is an accepted alternative. In recent years, significant improvements have been made in both the sensitivity and specificity of PCR assays used for the detection of Haemophilus influenzae. Real-time PCR assays are available to detect DNA of all six Haemophilus influenzae serotypes in blood, CSF, or other clinical specimens. A major advantage of PCR is that it allows for detection
of H. influenzae from clinical samples in which the organism could not be detected by culture
methods, such as when a patient has been treated with antibiotics before a clinical specimen is
obtained for culture. Even when the organisms are nonviable following antimicrobial treatment,
PCR can still detect H. influenzae DNA. Isolation of the bacterium is needed to test for antimicrobial susceptibility.
Although not widely available, subtyping the Hib bacterium by pulsed field gel electrophoresis
(PFGE),[16,17] and multilocus sequence typing (MLST) can be performed for epidemiologic
purposes. Some subtyping methods such as outer membrane proteins, lipopolysaccharides, and
enzyme electrophoresis are no longer recommended or performed because they were unreliable
or too labor intensive. The state health department may direct questions about subtyping to the
CDC Meningitis and Vaccine Preventable Diseases Branch laboratory at 404-639-3158.
For additional information see Chapter 22, "Laboratory Support for Surveillance of Vaccine-preventable
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Invasive H. influenzae disease became nationally notifiable in 1991. Each state and territory
has regulations or laws governing the reporting of diseases and conditions of public health
importance. These regulations and laws list the diseases to be reported and describe those
responsible for reporting, such as healthcare providers, hospitals, laboratories, schools, child
care facilities, or other institutions. Vaccine failure information should be collected for casepatients
who received all required doses of vaccines but still contracted Hib. CDC has a form
for reporting vaccine failures, or a state form can be used if available. Persons reporting should contact their state health department for state-specific reporting requirements. The Meningitis
and Vaccine-Preventable Diseases Branch, NCIRD, can be contacted during office hours,
8:00 a.m.-4:30 p.m. Eastern time, at 404-639-3158.
Reporting to CDC
A provisional report of probable and confirmed cases should be sent to the National Notifiable
Disease Surveillance System by the state health department via the National Electronic
Telecommunications System for Surveillance (NETSS) or the National Electronic Disease
Surveillance System (NEDSS), when available, within 14 days of the initial report to the state or
local health department (Appendix 4, Haemophilus influenzae Disease Surveillance Worksheet [1 page]). Reporting should not be delayed because of incomplete
information or lack of confirmation. Cases of disease should be reported by the state in which
the patient resides at the time of diagnosis.
The "Expanded Haemophilus influenzae type b Surveillance Worksheet" (Appendix 4) can be
used to collect information on each case. Many state health departments have the technology
available to send this detailed case report information to CDC through NETSS by using
supplemental data entry screens. States that do not have access to supplemental data entry
screens should contact CDC. The highest priority for completion of supplemental information
forms should be given to cases of H. influenzae invasive disease in children younger than
5 years of age. The second highest priority for completion of forms should be cases of
H. influenzae invasive disease in children 5–14 years of age.
Information to collect
The following data are epidemiologically important and should be collected in the course of
case investigation. Additional information may be collected at the direction of the state health
- Demographic information
- Date of birth
- Reporting Source
- Earliest date reported
- Case ID
- Date of illness onset
- Type of disease syndrome (meningitis, bacteremia, epiglottitis, pneumonia, arthritis,
osteomyelitis, pericarditis, cellulitis)
- Outcome (patient survived or died)
- Serotype of isolate
- Specimen source from which organism was isolated (blood, CSF, pleural fluid, peritoneal
fluid, pericardial fluid, joint fluid, amniotic fluid, or other normally sterile site)
- Date first positive culture identified as H. influenzae
- Date of specimen collection
- Antibiotic susceptibility
- Vaccination status (for type b or unknown serotype infections only)
- Dates of Hib immunization
- Manufacturer name
- Vaccine lot number
- If not vaccinated, reason
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Table 1 and Table 2 list the Hib monovalent and combination conjugate vaccines that are currently
available and the recommended vaccination regimens for each vaccine. The combination
vaccines include the Hib monovalent vaccine and vaccines for other vaccine-preventable
diseases in one vial, thus decreasing the number of injections needed for protection. Based
on the recommended vaccination schedule, infants should receive three primary doses of Hib
conjugate vaccine with PRP-T (monovalent or combination) at ages 2, 4, and 6 months, or two
primary doses of PRP-OMP (monovalent or combination) at 2 and 4 months. A booster dose
should be administered at age 12–15 months with any of the Hib vaccines. Any type of licensed
Hib vaccine may be used interchangeably to complete the series. However, the PRP-T Hiberix
vaccine is licensed only for use as a booster dose and should not be used for the primary series. The number of doses needed to complete the primary series is determined by the type of vaccine used; if a PRP-OMP vaccine is not administered as both doses in the primary series, a
third dose of Hib vaccine is needed to complete the series.[19–22] The recommended schedule for
Hib vaccination of previously unvaccinated children is shown in Table 3.
Table 1. Hib monovalent conjugate vaccines currently available and recommended regimens for routine vaccination of children in the United States.
|PRP-T (Sanofi Pasteur)
||2, 4, 6 months
|PRP-OMP (Merck & Co., Inc)
||2, 4 months
||Not licensed for primary series
Table 2. Combination vaccines currently available and recommended regimens for routine vaccination of children in the United States.
|PRP-OMP + HepB (Merck & Co., Inc)
||2, 4 months
|PRP-T + DTaP+IPV (Sanofi Pasteur)
||2, 4, 6 months
||2, 4, 6 months
Table 3. Recommended schedule for Hib conjugate vaccine administration among previously unvaccinated children*.
|Age at first dose
||2–3§ doses, 4 weeks apart
||At 12–15 months†**
||2 doses, 4 weeks apart
||At 12–15 months
||8 weeks later
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Elimination of childhood Hib disease requires participation by all levels of the healthcare
system so that all cases are identified and assessed rapidly and reported promptly, and data
on reported cases are used in an optimal manner to prevent disease among unvaccinated or
undervaccinated populations. The activities listed here can improve the detection and
reporting of cases as well as the completeness and quality of reporting. See Chapter 19, “Enhancing Surveillance,” for additional recommendations for enhancing surveillance of vaccine-preventable diseases.
Ensuring that all isolates from children are serotyped
Because of the need to make rapid decisions about chemoprophylaxis, serotype should be
determined and reported for all H. influenzae isolates. It is particularly important that serotype
be reported for cases in children younger than 5 years of age; the second highest priority is for
cases among children 5–14 years of age. This information is also used to determine whether
a case indicates a vaccine failure (i.e., a vaccinated person who gets the disease) or a failure
to vaccinate. The state public health laboratory or another reference laboratory should be able
to provide serotype testing of H. influenzae isolates. Hospital laboratories unable to perform
serotype testing should forward all H. influenzae isolates for serotyping to one of these
laboratories, or should contact the state health department for advice, if necessary.
Monitoring surveillance indicators
Regular monitoring of surveillance indicators, including reporting dates, time intervals between diagnosis and reporting, and completeness of reporting may identify specific areas of the surveillance system that need improvement. Important indicators to evaluate the completeness
and overall quality of the surveillance system include the following:
- Proportion of H. influenzae cases reported to NNDSS with complete information
(clinical case definition-species, specimen type; vaccine history; and serotype testing)
- Proportion of Hib cases among children younger than 5 years of age with complete
- Proportion of Hib cases among children younger than 5 years of age with serotyped isolate
Monitoring the incidence of invasive disease due to non-b H. influenzae
The epidemiology of invasive H. influenzae disease in the United States has shifted in the postvaccination era. Nontypable H. influenzae now causes the majority of invasive H influenzae disease in all age groups. Using data from active surveillance sites from 2004 through 2013, the estimated annual incidence of invasive nontypable-b and non-b H. influenzae disease in children younger than 5 years of age was 1.66/100,000 and 0.93/100,000, respectively (unpublished data). These rates may be used as a surveillance indicator for monitoring the completeness of invasive H. influenzae case reporting. Although limited data are available on temporal and geographic variability in incidence of nontypable and non-type-b invasive diseases, use of these surveillance indicators is encouraged.
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Laboratory, hospital, and clinic records should be reviewed during case investigations by
health department personnel in order to collect important information such as serotype,
immunization status, dates of vaccination, vaccine lot numbers, and clinical illness description
and outcome. The Expanded Haemophilus influenzae serotype b Surveillance Worksheet may
be used as a guide for collecting demographic and epidemiologic information in a case
investigation (see Appendix 4).
Identification of young children who are household or childcare contacts of patients with Hib invasive disease and assessment of their vaccination status may help identify persons who
should receive antimicrobial prophylaxis or who need to be immunized.
The Advisory Committee on Immunization Practices recommends that because children who
attend child care are at increased risk for Hib disease, efforts should be made to ensure that all
child care attendees younger than 5 years of age are fully vaccinated. Children < 24 months
of age who develop invasive Hib disease should repeat the Hib vaccine series because they can
remain at risk of a second episode of disease; children >24 months of age who develop invasive
Hib disease usually develop a protective immune response and do not need immunization. For
household contacts of a person with invasive Hib disease, no rifampin chemoprophylaxis is
indicated if all persons are 48 months of age or older, or if children younger than 48 months
of age are fully vaccinated according to the schedule in Table 3. Rifampin chemoprophylaxis is recommended for index case-patients (unless treated with cefotaxime or ceftriaxone) and all household contacts in households with households with members aged <4 years who are not fully vaccinated or members aged <18 years who are immunocompromised, regardless of their vaccination status. The recommended rifampin dose is 20 mg/kg as a single daily
dose (maximal daily dose 600 mg) for 4 days. A dose of 10 mg/kg once daily for 4 days is
recommended for neonates (less than 1 month of age).
The risk of Hib invasive disease for
child care center contacts of a patient with Hib invasive disease case is thought to be lower than
that for a susceptible household contact. Public health officials should refer to the American
Academy of Pediatrics (AAP) Red Book 2012 for information on chemoprophylaxis of child
care center contacts.
There are no guidelines for control measures around cases of invasive nontypable or non-b H. influenzae disease. Chemoprophylaxis is not recommended for contacts of persons with invasive disease caused by nontypable or non-b H. influenzae because cases of secondary transmission of disease have not been documented [23, 24].
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