Haemophilus influenzae type b (Hib)
Haemophilus influenzae type b (Hib) is a gram-negative bacterium that was once the leading cause of bacterial meningitis and other serious invasive diseases in children under 5 years of age, but has been dramatically reduced through effective vaccination programs. 1
Characteristics and Epidemiology
Haemophilus influenzae is a species of bacteria that exists in two main forms:
- Encapsulated (typeable): Expresses one of six distinct capsular polysaccharides (types a, b, c, d, e, or f)
- Unencapsulated (nontypeable): Lacks a capsule
Hib specifically refers to the type b encapsulated strain, which was historically the most virulent form. Before effective vaccines:
- 1 in 200 children developed invasive Hib disease by age 5 1
- 60% of cases presented as meningitis 1
- 3-6% of cases were fatal 1
- 20-30% of meningitis survivors experienced permanent sequelae ranging from mild hearing loss to mental retardation 1
Clinical Manifestations
Hib can cause several serious invasive diseases:
- Meningitis (inflammation of the brain and spinal cord membranes)
- Epiglottitis (severe throat infection that can block airways)
- Pneumonia (lung infection)
- Septic arthritis (joint infection)
- Cellulitis (skin infection)
- Purulent pericarditis (infection of the sac around the heart)
- Bacteremia (bloodstream infection) 1
Nontypeable H. influenzae strains more commonly cause:
- Otitis media (ear infections)
- Conjunctivitis (eye infections)
- Sinusitis (sinus infections) 1
Transmission and Risk Factors
- Colonizes the upper respiratory tract of humans
- Transmitted person-to-person through respiratory droplets or direct contact with respiratory secretions 1
- Highest risk in unimmunized or underimmunized children
- Children with certain immunocompromising conditions are at increased risk:
- Functional or anatomic asplenia
- HIV infection
- Immunoglobulin deficiencies
- Early component complement deficiency
- Recipients of hematopoietic stem cell transplants
- Recipients of chemotherapy or radiation therapy 1
Vaccine Development and Impact
The development of effective vaccines against Hib represents one of the great public health successes:
First-generation vaccines (1985): Pure polysaccharide vaccines with limited effectiveness, especially in children under 18 months 1
Conjugate vaccines (1987-1989): Conjugation of the PRP polysaccharide with protein carriers created T-cell dependent characteristics, making them effective even in young infants 1
Impact: Following widespread vaccination:
Current Vaccination Recommendations
ACIP recommends:
- Routine vaccination with conjugate Hib vaccine for infants aged 2-6 months
- Dosing schedule: 2 or 3 doses (depending on vaccine product)
- Booster dose: At 12-15 months of age
- Special populations: Vaccination for persons with certain immunocompromising conditions 1
Challenges and Future Directions
Despite the success of Hib vaccination programs:
- Disparities persist, with higher rates among American Indian/Alaska Native populations 1
- Emergence of other serotypes, particularly H. influenzae serotype a (Hia), has been observed in certain populations 3
- Achieving the national health objective of zero indigenous Hib cases in children under 5 requires continued vigilance 2
Clinical Implications
For healthcare providers:
- Consider Hib in the differential diagnosis for meningitis, epiglottitis, and other serious infections, especially in unvaccinated or incompletely vaccinated children
- Patients who develop Hib disease despite appropriate vaccination should be evaluated for immunological deficiencies 1
- Prompt management, reporting, and education about vaccination are crucial actions for clinicians 4
Hib vaccines only protect against H. influenzae type b strains; no vaccines against non-type b or nontypeable strains are currently available 1.