Epidemiology

Human cases were first identified in 1970 in Zaire (now known as the Democratic Republic of Congo, or DRC). Since then, cases have been increasing in humans over the past five decades, particularly in the DRC, which has reported cases continuously over that time. This increase in cases may be due to cessation of smallpox vaccination (which provided some cross-protection) or waning immunity, environmental factors (e.g., increased population density, human settlements in unknown animal reservoirs, deforestation), and/or genetic evolution of the virus, but there is no evidence to support these theories. The median age at presentation has evolved from young children (4 years) in the 1970s to young adults (21 years) in 2010 to 2019. Travel-associated spread outside of Africa has occasionally resulted in sporadic outbreaks in the UK, the US, Singapore, and Israel.[4][5]

Multi-country outbreak (2022)

  • In May 2022, clusters of cases were identified in several countries with no direct travel links to an endemic area, and the outbreak is currently ongoing. This is the first time that chains of transmission have been reported in countries without known epidemiologic links to West and Central Africa.[7] 

  • The earliest known case has a specimen date of 7 March 2022, and was identified through retrospective testing of a residual sample.[8]

  • Cases are predominantly in younger adult men who identify as gay, bisexual, or other men who have sex with men.[7] However, cases have also been reported rarely in women (including pregnant women) and children (including neonates).[9][10][11][12] In the US, children and adolescents ages <18 years represented 0.3% of all cases and none resulted in critical illness or death.[13]

  • Check your local public health authority resources for current information on this outbreak.

  • For current epidemiologic information see Summary (Important updates).

Endemic countries

  • The disease is endemic in Central and Western Africa. Cases have been reported across 12 countries (i.e., DRC, Nigeria, Benin, Central African Republic, Republic of the Congo, Sierra Leone, Cameroon, Côte d’Ivoire, Gabon, Liberia, Ghana, and South Sudan).[14] The majority of cases have been reported in rural areas (adjacent to or within tropical forests) of the Congo Basin and western Africa, particularly in the DRC where a major outbreak occurred from 1996 to 1997.[5]

  • Between 2000 to 2021, there have been 95 suspected and 40 confirmed outbreaks in the Central African Republic, with the number of outbreaks increasing after 2018.[15] In recent years, outbreaks have been reported in Nigeria (September 2017) and Cameroon (June 2018).[16][17] The outbreak in Nigeria is ongoing. The substantial resurgence in Nigeria appears to have been driven by a combination of factors, including population growth, increase in the number of unvaccinated people, and a decline in smallpox vaccine immunity.[18]

  • Since the beginning of 2022, 982 confirmed cases have been reported in Africa with 14 deaths as of 17 November 2022 (note: the World Health Organization has removed suspected cases and deaths from its previous tally, which included 1536 suspected cases and 72 deaths in the 10 June 2022 update).[6][19]

Travel-associated cases

  • UK: travel-associated cases were reported in September 2018 (three cases), December 2019 (one case), June 2021 (three cases), and May 2022 (one case). The three cases reported in September 2018 were the first ever reported in the UK. All cases had a travel history from Nigeria or were close contacts of travel-associated cases.[20][21][22][23][24][25]

  • US: an outbreak of over 70 cases occurred in the US Midwest in 2003, and was the first reported occurrence of the disease outside of Africa. The source of this outbreak was exposure to infected prairie dogs that had acquired the infection from Gambian rats imported from West Africa.[26] Since then, two travel-associated cases have been reported in July 2021 (Texas) and November 2021 (Maryland) in travelers returning from Nigeria.[27][28][29]

  • Singapore: one case was reported in May 2019 in a traveler from Nigeria.[30]

  • Israel: one case was reported in September 2018 in a traveler from Nigeria.[31]

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