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Although the types of infectious diseases caused by Haemophilus influenzae have changed considerably in recent years as a result of the widespread implementation of routine childhood immunization against type b organisms, this organism remains an important pathogen. There are two major categories of H influenzae: unencapsulated strains (untypeable, NTHi) and encapsulated strains (typeable). The unencapsulated strains are responsible chiefly for infections at mucosal surfaces, including conjunctivitis, otitis media, sinusitis, and bronchitis. In contrast, one of the six antigenically distinct encapsulated strains, strain type b, is associated with most invasive diseases such as septicemia, meningitis, cellulitis, septic arthritis, epiglottitis, and pneumonia. Prior to the availability of an effective vaccine, H influenzae type b (Hib) was the most common cause of pediatric bacterial meningitis in the United States.

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Epidemiology

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Humans are the only natural host for H influenzae. Maintenance of the organism in the human population depends on person-to-person transmission, which appears to occur by the respiratory tract to hand to respiratory tract route. This mode of transmission was best documented during nosocomial outbreaks of NTHi pneumonia in the elderly. Nontypeable strains colonize the upper respiratory tract of as many as 75% of healthy adults. Type b H influenzae (Hib) strains colonize the nasopharynx of children at a rate of 3% to 5%; the effectiveness of the conjugate vaccines is related (in part) to their ability to diminish the incidence of nasopharyngeal colonization (see below). Although both nontypeable and type b strains of H influenzae are easily spread via person-to-person transmission, only the Hib strains have historically been associated with invasive disease in children. Nasopharyngeal colonization by Hib is for the most part asymptomatic, but breakthrough bacteremia with subsequent development of focal infection was at one time a common occurrence and a major pediatric public health problem in the United States.

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In the prevaccine era, invasive Hib disease characteristically had a striking age-related incidence, with approximately 85% of disease occurring in children younger than 5 years. The peak incidence of the most serious form of invasive disease, meningitis, occurred between 6 and 12 months of age. Hib epiglottitis was, in contrast, predominantly a disease of older children, with more than 80% of the infections occurring in children older than 2 years. In the prevaccine era, approximately 20,000 instances of invasive Hib disease occurred annually in the United States, affecting about 1 in 200 children younger than 5 years.1

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Chronic illnesses associated with increased risk for invasive Hib disease include sickle cell disease, asplenia, agammaglobulinemia, trisomy 21, Hodgkin disease, and complement deficiencies. Increased risk has also been associated with childcare attendance, the presence of siblings younger than 5 years, household crowding, lower socioeconomic status, and passive smoke exposure. Breast-feeding confers some protection against disease. A bimodal seasonal disease pattern has been described, with one peak of illness in the autumn between September and December, and a second peak in the spring between March ...

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