Epidemiology in the Global Eradication of Smallpox

Henderson, D. A. (Smallpox Eradication Unit, W.H.O., Geneva). Epidemiology in the global eradication of smallpox. Int J. Epid. 1972, 1 : 25–30. During the past four years of the global smallpox eradication programme, smallpox incidence has declined from 131,000 cases in 1967 to 33,000 cases in 1970. The number of endemic countries is now less than half the number in 1967. While improved vaccine and vaccination techniques have played a significant role in the progress to date, the principal changes have occurred because of a better understanding of the epidemiology of smallpox and the application of surveillance procedures to interrupt transmission. In the endemic countries, the picture has emerged of a slowly spreading disease, affecting primarily those who have never been successfully vaccinated and transported from hut to hut or compound to compound by unvaccinated school-age children. At any one time, comparatively few villages or areas of a country are affected. Continuing transmission is, in major part, sustained by poorly vaccinated migrants in low socio-economic sectors of urban areas. With modification of the strategy in vaccination programmes to deal with high risk groups and with intensified surveillance and the institution of specific measures to interrupt the chains of transmission, dramatic changes in smallpox incidence have occurred within the past two years.