Seasonal influences on pertussis.

Both the absolute levels and the seasonal pattern of pertussis have changed over the past four decades in England and Wales. Three-to-four-year period epidemics tended to peak in the early or middle part of the year prior to 1957, during the last quarter between 1958 and 1975, and have been bimodal, with peaks in February and September, in both epidemics since 1976. Analysis of reported incidence data reveals similarities in the underlying seasonal pattern of transmission throughout this period. Seasonal effects include an increase in transmission coincident with the opening of schools in September. Other features of the pattern, such as a sharp fall in transmission in late November and a protracted low during February and March, are more difficult to explain, but may indicate important factors in the natural history of pertussis. The bimodality of recent epidemics appears to be a consequence of a seasonal low in transmission during February and March followed by a high level of transmission during the summer months. These seasonal trends may in turn reflect the increasing proportion of cases among pre-school children as a consequence of the fall in vaccine uptake during the mid 1970's.