Back-calculating the age-specific incidence of recurrent subclinical Haemophilus influenzae type b infection.

We consider the estimation of an age-specific incidence rate of a subclinical Haemophilus influenzae type b (Hib) infection from data recording the ages of children with a clinical Hib infection (Hib disease). The model is based on the assumption that the probability of being immune to clinical infection is determined by the time of the previous immunization caused by a subclinical infection, and by the distribution of the duration of immunity. We use a non-parametric Bayesian intensity model to arrive at smooth estimates of incidence rates. The estimated age-specific incidence rate of subclinical Hib infection is almost constant which indicates that the observed age-specific pattern of clinical Hib infection incidence is mainly due to immunity by either maternally derived antibodies or by immunizing subclinical infections. The estimated rate is relatively high, corresponding to one immunizing subclinical infection in less than two years.