Transmission of invasive Haemophilus influenzae type b disease in day care settings.

We prospectively studied the risk of secondary transmission of invasive Haemophilus influenzae type b (Hib) disease among children in day care in Oklahoma. We established a statewide surveillance system for the reporting of all forms of invasive Hib disease from physicians, clinical laboratories, county health departments, and infection control practitioners from all hospitals in and bordering Oklahoma. For the 2-year period March 15, 1984, to March 15, 1986, there were 409 culture-confirmed cases of invasive Hib disease in children 12 years of age or younger; 147 (37%) case-patients attended day care. When a case-patient attended day care during the week before illness, we prospectively identified and observed all classroom contacts (n = 2147) for the next 60 days. Of 1253 classroom contacts younger than 4 years of age, seven (0.6%) secondary cases were identified. Isolates from all paired index and secondary cases had identical outer membrane protein (OMP) patterns. Strains causing primary disease were indistinguishable by OMP distribution from those causing secondary disease. Of 685 classroom contacts younger than 2 years of age from whom rifampin use was ascertained, five (1.7%) of 292 who did not receive rifampin became secondary cases, compared with one (0.3%) of 393 who received rifampin (rate ratio 6.7; 95% confidence limits 1.1, 42.5; P less than 0.05). Of 495 classroom contacts 24 to 47 months of age for whom rifampin use was ascertained, one (0.5%) of 194 who did not receive rifampin became a secondary case, compared with none of 301 who received rifampin. This experience indicates that secondary transmission of invasive Hib disease can be appreciable in day care settings, particularly in children younger than 2 years of age, and that rifampin prophylaxis reduces this risk.