Measles in England and Wales--II: The impact of the measles vaccination programme on the distribution of immunity in the population.

Measles notification and vaccination data for England and Wales are analysed to reveal trends in age-specific incidence and immunity levels in successive cohorts born since 1950. A method for cumulating infection and vaccination experience is described in detail. The analysis reveals that there was a trend towards lower incidence rates of measles, and towards younger average age of cases, for the 10 years prior to commencement of the national immunization programme in 1968. Since the immunization programme began, there has been a dramatic and complicated shift in the age pattern of measles immunity in the population, such that some young age groups are now better protected, but some older age groups less well protected, than they were before the programme. These trends are discussed with critical attention to the possibility that they may reflect biases in the available data. It is concluded that the number of susceptibles over 10 years of age is increasing, and that this could have appreciable public health implications in the years to come. It appears that the total number of individuals susceptible to measles has remained relatively constant, between 4 and 4.5 million, before and after the immunization programme. Though consistent with basic theory, this maintenance of constant overall number of susceptibles has not been demonstrated before. Because the implications of these analyses are important for the strategy of measles control, it is important that appropriate serological surveys be carried out in order to assess the validity of the findings.