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

Abstract

Studies on the burden of human monkeypox in the Democratic Republic of the Congo (DRC) were last conducted from 1981 to 1986. Since then, the population that is immunologically naïve to orthopoxviruses has increased significantly due to cessation of mass smallpox vaccination campaigns. To assess the current risk of infection, we analyzed human monkeypox incidence trends in a monkeypox-enzootic region. Active, population-based surveillance was conducted in nine health zones in central DRC. Epidemiologic data and biological samples were obtained from suspected cases. Cumulative incidence (per 10,000 population) and major determinants of infection were compared with data from active surveillance in similar regions from 1981 to 1986. Between November 2005 and November 2007, 760 laboratory-confirmed human monkeypox cases were identified in participating health zones. The average annual cumulative incidence across zones was 5.53 per 10,000 (2.18-14.42). Factors associated with increased risk of infection included: living in forested areas, male gender, age < 15, and no prior smallpox vaccination. Vaccinated persons had a 5.2-fold lower risk of monkeypox than unvaccinated persons (0.78 vs. 4.05 per 10,000). Comparison of active surveillance data in the same health zone from the 1980s (0.72 per 10,000) and 2006-07 (14.42 per 10,000) suggests a 20-fold increase in human monkeypox incidence. Thirty years after mass smallpox vaccination campaigns ceased, human monkeypox incidence has dramatically increased in rural DRC. Improved surveillance and epidemiological analysis is needed to better assess the public health burden and develop strategies for reducing the risk of wider spread of infection.

Fig. 1.

Typical clinical presentation of human monkeypox in a 7-y-old female child, Sankuru District, Democratic Republic of Congo.

Fig. 2.

Map of health zones with active surveillance for human monkeypox, designated by dominant ecological characteristics, Sankuru District, Democratic Republic of Congo: 2006–2007. Zones are shaded by dominant ecological characteristics: dark green, heavily forested zone; red, ecotone (forest–savannah mosaic); yellow, savannah.

Fig. 3.

Average annual cumulative incidence of human monkeypox, by health zone and dominant ecological characteristic, Sankuru District, Democratic Republic of Congo: 2006–2007. Cumulative incidence and 95% CIs are shown for each major geographical area. Ecotone is a transitional zone of mosaic forests interspersed with stretches of savannah between savannah and forest regions.

Fig. 4.

Comparison of average annual cumulative incidence of human monkeypox by age group Kole Health Zone, Demographic Republic of Congo: 1981–86 vs. 2006–7. *, proportion of the population vaccinated in 2006–7 and in 1981–6 based on vaccination scar surveys in 1981–6 and in 2006. ^, vaccination rate steadily declined from 41.0% in 1981 to 4% in 1985 (13, 33).