Data on monkeypox in Zaire over the five years 1980-1984 are analysed to assess the protection imparted by past smallpox vaccination and the transmission potential of the virus in unvaccinated communities. Attack rates in individuals with and without vaccination scars indicated that smallpox vaccination (discontinued in 1980) imparted approximately 85% protection against monkeypox. It is predicted that monkeypox virus will continue to be introduced into human communities from animal sources, and that the average magnitude and duration of monkeypox epidemics will increase as vaccine-derived protection declines in the population. On the other hand, current evidence indicates that the virus is appreciably less transmissible than was smallpox, and that it will not persist in human communities, even in the total absence of vaccination. The findings thus support the recommendation of the Global Commission for the Certification of Smallpox Eradication to cease routine smallpox vaccination in monkeypox endemic areas, but to encourage continued epidemiological surveillance.

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With the eradication of smallpox, systematic routine vaccination with vaccinia has ceased and an increasing proportion of the human population in tropical rain forest areas of central and western Africa lacks vaccinia-derived immunity to monkeypox virus. This raises the question of the ability of monkeypox virus to establish and maintain itself in an unvaccinated population through continuous man-to-man transmission. A computerized stochastic model of Monte Carlo type was constructed to assess this potential risk. Simulated series were repeated 100 times to obtain distributions of predicted outcomes for decreasing levels of vaccination coverage (70 per cent, 50 per cent, and 0 per cent). The results revealed a substantial increase in new secondary cases in the total absence of vaccinia-induced immunity. Nevertheless, none of the simulated series did lead to an "explosive" epidemic. The model clearly indicated diminishing numbers of cases in successive generations and eventual cessation of transmission. Therefore, it appears highly improbable that the virus could maintain itself permanently in communities by interhuman transmission. After the eradication of smallpox, human monkeypox constitutes the most important orthopoxvirus infection in man, but analysis of information collected up to this time suggests that it does not represent currently a serious public health problem or a challenge to the achieved eradication of smallpox.

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