What is HIB (Haemophilus influenzae type b)?

Haemophilus influenzae type b (Hib)

Haemophilus influenzae type b (Hib) is an encapsulated gram-negative bacterium that was the leading cause of invasive bacterial disease in children under 5 years of age before effective vaccines were introduced, causing meningitis, epiglottitis, pneumonia, and other serious infections. 1

Epidemiology and Disease Burden

• Before effective vaccines, 1 in 200 children developed invasive Hib disease by age 5 2, 1
• 60% of these cases involved meningitis with a 3-6% mortality rate 2
• 20-30% of meningitis survivors experienced permanent sequelae ranging from mild hearing loss to mental retardation 2, 1
• Approximately two-thirds of all Hib disease cases affected infants and children less than 15 months of age 2
• Since vaccine introduction, there has been a 99% reduction in invasive Hib disease in children under 5 years 1

Clinical Manifestations

Hib can cause several serious invasive diseases:

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

In contrast, nontypeable H. influenzae strains more commonly cause:

• Otitis media
• Conjunctivitis
• Sinusitis 1

Pathogenesis

• The polyribosylribitol phosphate (PRP) capsule of Hib is a major virulence factor 2
• Antibody to PRP is the primary contributor to serum bactericidal activity 2
• Increasing levels of antibody are associated with decreasing risk of invasive Hib disease 2
• The human immune response to PRP resembles T-cell independent antigens, which explains the poor immune response in infants under 18 months 2

Prevention through Vaccination

• Conjugate Hib vaccines have dramatically reduced disease incidence worldwide 1
• Three different Haemophilus b conjugate vaccines were licensed in the early 1990s 2
• Conjugation of PRP polysaccharide with protein carriers enhances immunogenicity in young infants 1
• The Advisory Committee on Immunization Practices (ACIP) recommends:
  • Routine vaccination with conjugate Hib vaccine for infants aged 2-6 months
  • 2-3 primary doses (depending on vaccine type)
  • A booster dose at 12-15 months 1

Important 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 1
• Patients who develop Hib disease despite appropriate vaccination should be evaluated for immunological deficiencies 1
• Accurate identification of H. influenzae type b requires laboratory confirmation, as clinical presentation alone is insufficient 1
• PCR targeting capsule-specific genes and serotyping are considered gold standards for identifying Hib 1

High-Risk Populations

• American Indian/Alaska Native populations historically have had higher rates of Hib disease than the general population 1
• Children in day-care settings and household contacts of infected individuals are at increased risk 3
• Children with anatomic or functional asplenia or malignancies may be at higher risk 3

Cautions and Pitfalls

1. Diagnostic pitfalls: Do not rely solely on clinical presentation or standard taxonomy ID, as these cannot distinguish between different H. influenzae serotypes 1

2. Vaccination limitations: Hib vaccines only protect against H. influenzae type b strains; no vaccines against non-type b or nontypeable strains are currently available 1

3. Testing interpretation: Positive antigen test results from urine or serum samples are unreliable for diagnosis of H. influenzae disease 1

4. Prophylaxis considerations: Index patients should receive prophylactic rifampin therapy before hospital discharge, as IV antibiotics may cure the systemic infection but allow nasopharyngeal colonization to persist 3