Distribution of immunity to pertussis in the population of England and Wales.

This paper presents three different methods of estimating the number and age distribution of individuals susceptible to pertussis in England and Wales. The first approach is an extrapolation from data in the prevaccine era, the second is based upon theoretical consideration of the transmission dynamics of pertussis, and the third is a detailed cohort analysis of available notification and vaccination data. Each of these analyses suggests that the total number of people susceptible to pertussis infection in England and Wales has remained at 3-4 million for the past 40 years, despite the increase and changes in numbers vaccinated. The effect of vaccination has been to reduce the incidence of infection and disease, but not to reduce the number of susceptibles. These findings, which are consistent with ''mass action theory'', could be tested by an appropriately designed seroepidemiological survey.