Is Haemophilus influenzae Type b (HIB) Part of Normal Flora?
Yes, Haemophilus influenzae type b can colonize the upper respiratory tract of humans as part of the normal flora, but this colonization is the source of potential invasive disease, particularly in unvaccinated individuals. 1
Colonization and Transmission
H. influenzae colonizes the upper respiratory tract (oropharynx) of humans and is transmitted person-to-person by inhalation of respiratory droplets or by direct contact with respiratory tract secretions. 1
The organism is a strict parasite of humans found principally in the upper respiratory tract, making humans the only natural reservoir. 2
Historically, before widespread Hib vaccination, colonization rates were substantially higher, particularly among young children who served as the primary reservoir for transmission. 1
Impact of Vaccination on Colonization
The introduction of Hib conjugate vaccines has dramatically reduced both invasive disease (by 99%) and oropharyngeal colonization of Hib in vaccinated populations. 1
Vaccination leads to decreases in oropharyngeal carriage, which has contributed to herd immunity and protection of unvaccinated individuals. 1
The Hib vaccine has had minimal impact on mucosal disease caused by nontypeable H. influenzae strains, which continue to colonize the respiratory tract and cause conditions like otitis media and sinusitis. 1
Clinical Significance of Colonization
While colonization itself is asymptomatic, it represents the critical first step toward invasive disease, particularly in children under 5 years of age who lack protective antibodies. 3
In the pre-vaccine era, approximately 1 in 200 children developed invasive Hib disease before age 5 years, demonstrating that colonization could progress to life-threatening infections including meningitis, epiglottitis, and septicemia. 1, 3
Rifampin chemoprophylaxis can be effective in eliminating Hib carriage and preventing invasive disease in household contacts of infected individuals. 1
Important Distinctions
Encapsulated H. influenzae type b strains (which colonize and cause invasive disease) must be distinguished from nontypeable H. influenzae strains, which more commonly colonize the respiratory tract and primarily cause mucosal infections. 1
Type b strains account for greater than 95% of systemic infections in unvaccinated children, while nontypeable strains more commonly cause otitis media and sinusitis without bacteremia. 2