The antigenic variation of influenza A 2 virus and the antibody present in the population to different variants are described and related to the occurrence of epidemics of influenza in England since 1957.

The occurrence of epidemics of influenza in Britain since 1957, when the A 2 (Asian) virus was first introduced, has been traced by an analysis of influenza deaths and sickness insurance benefit claims. In a more detailed study of the epidemic in 1967–8, the largest for 7 years, mortality statistics, sickness benefit claims, consultation rates in general practice and laboratory reports were analysed according to time and geographical location, the age distribution of cases and their clinical features. The value of these various indices in the surveillance of influenza is discussed. It is estimated that the number of cases of influenza during the 1967–8 epidemic was just over one million in the working population and almost two millions in the whole population of Britain.

We wish to thank Dr D. L. Crombie, Honorary Director of the Royal College of General Practitioners' Records and Statistics Unit for permission to quote from returns compiled by the Unit and Mrs P. Jones who extracted the information; we also thank Dr A. M. Adelstein of the General Register Office and Dr J. R. Connelly and Dr A. T. Roden of the Department of Health and Social Security for their helpful comments, and the many laboratory directors whose findings are reported in the Communicable Disease Report of the PHLS. We are also much indebted to Mrs R. Narasimhan who helped with the analysis of the data.