Stochastic Model for Interhuman Spread of Monreypox

Abstract

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.

Vacated niches, competitive release and the community ecology of pathogen eradication

“…All observed transmission chains were short, but their results showed that past smallpox vaccination gave strong cross-protective immunity against monkeypox, leading to concern that monkeypox might establish sustained transmission in an unvaccinated population. In a pioneering analysis, Jezek et al [48] extrapolated their findings to show that-if nothing changed except the proportion vaccinated in the population-monkeypox would not establish sustained transmission among humans. A follow-up article re-stated this finding in terms of the basic reproductive number, reporting that R 0 for monkeypox in an unvaccinated population (in rural DRC) was approximately 0.83, less than the threshold value of 1 needed for persistent circulation [23].…”

Animal Models of Orthopoxvirus Infection

“…However, a WHO-sponsored epidemiological investigation of an upsurge of monkeypox cases during 1996 and 1998 in the Democratic Republic of Congo indicated that secondary transmission (person to person), rather than primary transmission (animal to human), accounted for the majority of those cases. 16,17,56,85,140 Monkeypox in humans is similar to classic variola major and is characterized by exanthema, fever, and respiratory signs. In contrast to smallpox, human monkeypox has a lower case mortality rate (approximately 10%), and there is striking peripheral lymphadenopathy, usually involving cervical and inguinal lymph nodes.…”

Major increase in human monkeypox incidence 30 years after smallpox vaccination campaigns cease in the Democratic Republic of Congo

“…This intensive surveillance accounted for 338 of the 404 recognized cases in Africa during this period (13). Epidemiologic data from this program was used to create a stochastic model for spread of monkeypox between humans, which indicated that monkeypox virus was highly unlikely to sustain itself in human populations and, therefore, did not constitute a major public health problem (14,15). This modeling analysis used available information to predict the future of monkeypox dynamics when the population was completely unvaccinated, but did not include statistical uncertainties and could not account for changes in the ecological reservoir and subsequent epidemiology.…”