Epidemiology of Haemophilus influenzae type B disease among Navajo Indians

During a 7-year period ending June 30, 1980, the annual incidence of all Haemophilus influenzae type b disease among Navajo children less than 5 years old was 214 per 100,000, and that of H. influenzae meningitis was 152 per 100,000. Eighty-one percent of H. influenzae meningitis occurred in children 12 months of age or younger, and 64 percent clustered in children ages 4 through 8 months. Meningitis accounted for 70 percent of all invasive disease. No epiglottitis was observed. The epidemiology is similar to that in Yupik Eskimos, who have an even higher rate of H. influenzae type b disease than Navajos but are a much smaller population. Mortality from H. influenzae meningitis was low (4 percent) among Navajo children, but neurological sequelae were observed in at least 16 percent of the survivors. This high rate of sequelae may be due in part to clustering of cases in infancy. Among normal Navajo neonates, 79 percent had maternal capsular type b antibody titers greater than or equal to 0.15 micrograms per deciliter (microgram per dl), and the whole group had a geometric mean titer of 0.51 micrograms per dl. By age 4 months, when meningitis cases became frequent, only 14 percent of Navajo infants had antibody titers greater than or equal to 0.15 micrograms per dl. Twelve of 67 asymptomatic infants (18 percent), each monitored every 2 months, had H. influenzae type b or a cross-reacting organism isolated from the pharynx on at least one occasion before they were 9 months old. Active immunization would be theoretically indicated in this population with high H.influenzae type b exposure and disease, but a vaccine would have to confer substantial immunity in very young infants.