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 was previously known to be endemic in Central and Western Africa.[7]​ The majority of cases were 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 and 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 in 2017 after 39 years without reports of cases. Since then, five outbreaks have occurred between 2017 and 2021.[9]

  • The largest outbreak of confirmed cases in the Central African Republic occurred in November 2021, with 14 cases and four waves of interhuman transmission.[10]

  • Outbreaks occurred in refugee camps in the Republic of Sudan in 2022, with no zoonotic origin reported.[11]

Ongoing global clade II mpox outbreak

  • A global outbreak of clade II mpox was first identified in May 2022 and is ongoing across many countries. Clusters of cases were initially 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.

  • The WHO declared that the outbreak constituted a Public Health Emergency of International Concern (PHEIC) in July 2022. This first PHEIC ended in May 2023 as the frequency of cases had decreased significantly, although cases continue to be reported. The group of viruses largely circulating in the outbreak belong to clade IIb.

  • Since the beginning of 2022, over 115,000 laboratory-confirmed cases have been reported globally from 126 countries, including 255 deaths, as of 31 October 2024. The most affected countries include the US, Brazil, the DRC, Spain, France, and Colombia.[12]​ Sporadic case clusters have been reported in locations where transmission had previously ceased.[13]

  • Unlike previous outbreaks, cases outside of Africa have predominantly been in younger adult men, most of whom identify as gay, bisexual, or other men who have sex with men.[14][15][16]​​ There is no signal suggesting sustained transmission outside of these networks. Males ages between 18 and 44 years have been disproportionately affected.[12] 

  • Cases have also been reported in women (mainly heterosexual, including pregnant women) and children, albeit more rarely.[17][18][19]​​ In the US, 2.7% of all reported cases were in women including 3% in pregnant or recently pregnant women (as of November 2022).[20]

  • In Nigeria, there were more cases in children and women, and fewer cases in people without HIV infection and gay, bisexual, or other men who have sex with men, compared to countries outside of Africa. Hospitalization rates were also much higher compared with other countries.[21]

  • Transgender and gender-diverse people have been disproportionately affected during the outbreak.[22][23]​​ Racial and ethnic disparities also exist.[24]

  • Adults ages >50 years appear to have a similar risk of hospitalization and death compared with adults ages <50 years, possibly due to older adults receiving the smallpox vaccine as part of their childhood immunization schedule.[25]

Clade I mpox outbreak originating in Africa

  • ​​​​There has been a significant increase in the number of cases and deaths in the DRC throughout 2023 and 2024, with geographic expansion into previously unaffected areas (including neighboring countries such as Burundi, Kenya, Rwanda, and Uganda), and sexual transmission due to the clade Ib virus (a newly identified variant) documented for the first time.[12][28][29]​​​ The WHO declared that the outbreak constituted a PHEIC on the 14th August 2024 (reaffirmed on 28 November 2024).[30]​​

  • Approximately 15,485 laboratory-confirmed cases of mpox (both clades), including 78 deaths, have been reported in Africa since the beginning of 2022 (as of 2 December 2024). Of these cases, 13,081 were reported in 2024, including 55 deaths, across 20 countries. The three countries with the majority of cases in 2024 are the DRC, Burundi, and Uganda.[12] The number of suspected cases is much higher; these cases remain untested due to limited diagnostic capacity in some African countries.

  • The outbreak is ongoing, with sustained community transmission driven by sexual transmission. It is not currently known whether the circulating clade Ib variant is more transmissible or causes more severe disease compared with other clade I variants.[31][32]

  • In the DRC, 67% of cases of clade Ib mpox were in people ages 15-30 years, while those <15 years of age only compromised 15% of cases.[33]​ However, in Burundi, almost half of the confirmed clade Ib cases were in children <15 years of age, and 30% were between 15-29 years of age.[34]

  • A small number of cases of travel-associated clade Ib mpox have been reported outside of Africa (e.g., India, Sweden, Thailand, Germany, the US, the UK).[26][27]​[35][36]​​[37]​​​​​​​

​Current epidemiologic updates are available from public health authorities.

Previous outbreaks mainly affected children. However, the 2022 global clade II mpox outbreak has primarily affected male adults.[38]​ 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 clade II mpox outbreak, cases have been uncommon in children compared with adults.​[15][19]​​​​​​​​​ Globally, 1.3% of cases were in children and adolescents ages <18 years of and no deaths were reported.[39]​ In the US, children and adolescents ages <18 years represented <0.01% of all cases and none resulted in critical illness or death. Cases have been reported rarely in neonates.​[40]​​

Available data from the 2022 global clade II mpox outbreak indicated that 28% to 51% of patients have been HIV-positive (in cases where HIV status is known).[41][42]​​​​​ Although women account for a minority of cases, the proportion of women with HIV has been estimated to be 27% (50% in transgender women).[43]​ However, in the largest study of women to date, only 4.4% of women had HIV (compared with 40.8% in men).[18]​ 

Prior to the 2022 global clade II mpox outbreak, a small number of travel-associated cases had been reported in the UK, the US, Singapore, and Israel.[44][45]​​​​​[46]​​[47][48][49][50][51][52]​​​ 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.[53] ​

Use of this content is subject to our disclaimer