Haemophilus influenzae
: Secular Trends in the United States
H. influenzae
infections became nationally reportable in 1991. Serotype-specific
reporting continues to be incomplete.
Before the availability of national reporting data, several areas conducted
active surveillance for H. influenzae disease, which allowed
estimates of disease nationwide. In the early 1980s, it was estimated that
about 20,000 cases occurred annually in the United States, primarily among
children younger than 5 years of age (40–50 cases per 100,000 population).
The incidence of invasive Hib disease began to decline dramatically in the
late 1980s, coincident with licensure of conjugate Hib vaccines, and has
declined by more than 99% compared with the prevaccine era.
From 2003 through 2010, an average of 2,562 invasive H. influenzae infections per year were reported to CDC in all age
groups (range 2,013–3,151 per year). Of these, an average of 398
(approximately 16%) per year were among children younger than 5 years of
age. Serotype was known for 52% of the invasive cases in this age group.
Two-hundred-two (average of 25 cases per year) were due to type b. In 2011,
among children younger than 5 years of age, 14 cases of invasive disease
due to Hib were reported in the United States. An additional 13 cases of
Hib are estimated to have occurred among the 226 reports of invasive H. influenzae infections with an unknown serotype.
During 2010-2011, 33% of children younger than 5 years of age with confirmed invasive Hib disease were younger than 6 months of age and too young to have completed a three-dose primary vaccination series. Sixty-seven percent were age 6 months or older and were eligible to have completed the primary vaccination series. Of these age-eligible children, 64% were either unvaccinated, incompletely vaccinated (fewer than 3 doses), or their vaccination status was unknown. Thirty-six percent of children aged 6–59 months with confirmed type b disease had received three or more doses of Hib vaccine, including 5 who had received a booster dose 14 or more days before onset of their illness. The cause of Hib vaccine failure in these children is not known.
Risk factors for Hib disease include exposure factors and host factors that increase the likelihood of exposure to Hib. Exposure factors include household crowding, large household size, child care attendance, low socioeconomic status, low parental education levels, and school-aged siblings. Host factors include race/ethnicity (elevated risk among Hispanics and Native Americans—possibly confounded by socioeconomic variables that are associated with both race/ethnicity and Hib disease), chronic disease (e.g., sickle cell anemia, antibody deficiency syndromes, malignancies – especially during chemotherapy), and possibly gender (risk is higher for males).
Protective factors (effect limited to infants younger than 6 months of age) include breastfeeding and passively acquired maternal antibody.
Secondary Hib disease is defined as illness occurring 1–60 days following contact with an ill child, and accounts for less than 5% of all invasive Hib disease. Among household contacts, six studies have found a secondary attack rate of 0.3% in the month following onset of the index case, which is about 600-fold higher than the risk for the general population. Attack rates varied substantially with age, from 3.7% among children 2 years of age and younger to 0% among contacts 6 years of age and older. In these household contacts, 64% of secondary cases occurred within the first week (excluding the first 24 hours) of disease onset in the index patient, 20% during the second week, and 16% during the third and fourth weeks.
Data are conflicting regarding the risk of secondary transmission among child care contacts. Secondary attack rates have varied from 0% to as high as 2.7%. Most studies seem to suggest that child care contacts are at relatively low risk for secondary transmission of Hib disease particularly if contacts are age-appropriately vaccinated.
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