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Measles in England and Wales--I: An analysis of factors underlying seasonal patterns.

Examination of the weekly measles notifications for England and Wales, 1950-1979, reveals a regular biennial pattern of major and minor epidemics before the national immunization programme began in 1968, followed by an annual cycle of minor epidemics. Each year the reported incidence reaches its annual low between weeks 36 and 39, very close to the opening of primary schools. Analysis of these data with a simple mass action model reveals that the underlying transmission parameter has had a similar annual pattern in years of major and minor epidemics. The transmission parameter rises three times each year, coinciding with opening of school terms, and falls with school term and mid-term holidays. This pattern of the transmission parameter has been maintained in the decade since national vaccination began, indicating that the importance of schools in the annual dynamics of measles has not changed. The analysis further suggests that the national measles vaccination programme has not lowered the total number of individuals susceptible to measles in England and Wales.