A day care-based study of the efficacy of Haemophilus b polysaccharide vaccine.

To assess the efficacy of the Haemophilus b polysaccharide vaccine following licensure and to evaluate the risk of Haemophilus influenzae type b (Hib) disease in the week following vaccination, we conducted a day care-based case-control efficacy study using cases of invasive Hib disease ascertained through active surveillance in areas with a total population of 34 million. For each patient 18 to 59 months old, up to three 18- to 59-month-old controls were chosen from the same day care classroom. Using conditional logistic regression, the vaccine efficacy was estimated to be 45% (95% confidence interval = -1% to 70%) and did not change significantly after accounting for potential biases. In addition, three (3%) of 104 patients vs five (2%) of 207 controls were vaccinated within seven days before the patients' dates of admission (odds ratio = 1.8, 95% confidence interval = 0.3 to 10.2), which does not suggest an increased risk of Hib disease in the week following immunization. This study suggests that the efficacy of the currently used HBPV is less than expected from previous studies and points out the usefulness of case-control studies for monitoring vaccine efficacy following licensure.