What is Haemophilus influenzae type b (Hib)?

Haemophilus influenzae type b (Hib)

Haemophilus influenzae type b (Hib) is a gram-negative bacterium with a polysaccharide capsule 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 Classification

Haemophilus influenzae is a species of bacteria that exists in two main forms:

• Encapsulated (typeable) strains: Express one of six antigenically distinct capsular polysaccharides (types a, b, c, d, e, or f)
• Unencapsulated (nontypeable) strains: Lack a capsule 2

Hib specifically refers to the type b encapsulated strain, which contains the polyribosylribitol phosphate (PRP) capsule that serves as a major virulence factor for the organism. 2

Disease Burden and Epidemiology

Before effective vaccines were introduced:

• 1 in 200 children developed invasive Hib disease by age 5 years
• 60% of these children had meningitis
• 3-6% died from their infections
• 20-30% of meningitis survivors experienced permanent sequelae ranging from mild hearing loss to mental retardation 2, 1

Currently:

• Following widespread vaccination, there has been a 99% reduction in invasive Hib disease in children under 5 years
• Incidence remains below the Healthy People 2020 goal of 0.27/100,000
• Hib carriage has decreased from 2-7% in the prevaccine era to <1% 2, 1

Clinical Manifestations

Hib can cause several serious invasive diseases:

• Meningitis (most common presentation)
• Epiglottitis
• Pneumonia
• Septic arthritis
• Cellulitis
• Purulent pericarditis
• Bacteremia 1

Nontypeable H. influenzae strains more commonly cause:

• Otitis media
• Conjunctivitis
• Sinusitis 2

Transmission

• Hib colonizes the upper respiratory tract of humans
• Transmission occurs person-to-person through:
  • Inhalation of respiratory droplets
  • Direct contact with respiratory tract secretions 2, 1

High-Risk Populations

The majority of Hib disease occurs in:

• Unimmunized and underimmunized infants and children
• Infants too young to have completed the primary immunization series 2

Persons with certain immunocompromising conditions are at increased risk:

• Functional or anatomic asplenia
• HIV infection
• Immunoglobulin deficiency (including IgG2 subclass deficiency)
• Early component complement deficiency
• Recipients of hematopoietic stem cell transplants
• Recipients of chemotherapy or radiation therapy for malignant neoplasms 2, 1, 3

American Indian/Alaska Native populations historically have had higher rates of Hib disease than the general U.S. population, with disparities persisting even in the post-vaccine era. 2

Prevention through Vaccination

The development of effective conjugate vaccines against Hib represents one of the great public health successes:

• Early vaccines: The first Hib vaccine licensed in 1985 consisted of purified PRP capsular material, but had variable effectiveness and was ineffective in children under 18 months 2, 4

• Conjugate vaccines: Conjugation of the PRP polysaccharide with protein carriers confers T-lymphocyte-dependent characteristics to the vaccine, enhancing immunogenicity in young infants 2

• Current recommendations: The Advisory Committee on Immunization Practices (ACIP) recommends routine vaccination with conjugate Hib vaccine for:

  • Infants aged 2-6 months (with 2-3 primary doses)
  • A booster dose at 12-15 months
  • Vaccination for persons with certain immunocompromising conditions 1

Clinical Considerations

• Healthcare providers should 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 2, 1
• Hib vaccines only protect against H. influenzae type b strains; no vaccines against non-type b or nontypeable strains are currently available 2

Conclusion

Hib vaccination has dramatically reduced the incidence of invasive disease, but continued vigilance is required to maintain these gains, particularly in high-risk populations and settings with incomplete vaccination coverage.