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, 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. However, there is no evidence to support these theories.[4][5][6]

The disease is endemic in Central and Western Africa. 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 were 95 suspected and 40 confirmed outbreaks in the Central African Republic, with the number of outbreaks increasing after 2018.[8] Nigeria experienced a reemergence of cases caused by the Clade II virus in 2017 after 39 years without reports of cases. Since then, five outbreaks have occurred between 2017 and 2021.[9]

There is currently an ongoing global outbreak, which was first identified in May 2022. Clusters of cases were reported across several countries outside of Africa with no direct travel links to an endemic area. This was the first time that chains of transmission were reported in countries without known epidemiologic links to West and Central Africa.

  • Since the beginning of 2022, over 84,000 confirmed cases have been reported globally from 110 countries with 81 deaths, as of 20 January 2023. The number of cases peaked in August 2022 and has since been steadily declining. The most affected countries include the US, Brazil, Spain, France, and Colombia.[7] 

  • Cases have predominantly been in younger adult men, most of whom identify as gay, bisexual, or other men who have sex with men.[10][11][12] Males ages between 18 and 44 years are disproportionately affected.[7] 

  • Cases have also been reported in women (mainly heterosexual, including pregnant women) and children, albeit more rarely.[13][14][15]​ In the US, 2.7% of all reported cases were in women including 3% in pregnant or recently pregnant women (as of November 2022).[16]

  • Transgender and gender-diverse people have been disproportionately affected during the outbreak.[17]

​​Since the beginning of 2022, 1214 confirmed cases and 15 deaths have been reported in Africa as part of the outbreak, as of 20 January 2023. This represents 1% and 19% of global cases and deaths, respectively. Demographics of cases have been similar to recent previous outbreaks in Africa, but significantly different from other regions (65% of cases are in males with a median age of 25 years, and there are no data on sexual transmission).[7]

Current epidemiologic updates are available from public health authorities.

The median age at presentation has evolved from young children (4 years) in the 1970s to young adults (21 years) between 2010 and 2019.[4] In the 2022 global outbreak, cases have been uncommon in children compared with adults.​[11][15]​​​ For example, in the US, children and adolescents ages <18 years represented 0.3% of all cases and none resulted in critical illness or death.[18]

Available data from the 2022 global outbreak indicated that 28% to 51% of patients have been HIV-positive (in cases where HIV status is known).[19][20]​​​ Although women account for a minority of cases, the proportion of women with HIV has been estimated to be 27% (50% in transgender women).[21]​ However, in the largest study of women to date, only 4.4% of women had HIV (compared with 40.8% in men).[14]​ 

Prior to the 2022 global outbreak, a small number of travel-associated cases had been reported in the UK, the US, Singapore, and Israel.[22][23]​​​[24]​​[25][26][27][28][29][30]​ 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.[31] ​

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