Mpox Rapid Risk Assessment - Aug 2024

At a glance

CDC assessed the risk to the United States posed by the clade I mpox outbreak in the Democratic Republic of Congo and neighboring countries. The risk to the general population is assessed as low. The risk to men who have sex with men with more than one sexual partner, and their partners, is assessed as low to moderate.

Update

See our updated risk assessment here‎

Risk to United States from clade I mpox outbreak in Central and Eastern Africa

As of August 30, 2024

CDC assessed the risk to the United States posed by the clade I mpox outbreak in the Democratic Republic of Congo (DRC) and nearby countries for two populations.

  1. The risk to the general population is assessed as LOWA
  2. The risk to gay, bisexual, and other men who have sex with men (MSM) who have more than one sexual partner, and people who have sex with MSM, regardless of gender, is assessed as LOW to MODERATE

The purpose of this assessment is to provide public health agencies time-sensitive information about the ongoing clade I outbreak of mpox in the DRC and neighboring countries.

This assessment is an update to our July 8, 2024, risk assessment, in response to the recent spread of clade I mpox in Central and Eastern Africa and the declaration of a public health emergency of international concern on August 14, 2024. This update incorporates new evidence and the risk for the general population is now assessed as low, higher than previous assessments in which risk for the general population was assessed as very low. The risk assessment for MSM remains as low to moderate.

We have low confidence in this qualitative assessment (decreased from moderate level), due to uncertainty around the reasons for recent spread of the clade I mpox outbreak in Central and Eastern Africa, and implications of this spread for the United States. The assessment relies on subject-matter experts evaluating a range of evidence related to risk, including limited epidemiologic data from outbreaks in DRC and neighboring countries, historical data on clade I mpox epidemiology and clinical severity, simulation modeling results, and extrapolations based on the ongoing clade IIb mpox outbreak in the United States. We continue to monitor the outbreaks of clade I mpox in DRC and neighboring countries, and will update this risk assessment if new information warrants changes.

Risk assessment for general population in the United States

Overall Risk

We assess the overall risk to the general U.S. population as low, with low confidence. Overall risk is assessed by considering the likelihood and impact of infection across the population (see Methods section). This assessment may change as new evidence becomes available.

Likelihood

We assess the likelihood of infection for the general population as very low. Factors that informed our assessment of likelihood included the following:

There are no known cases of clade I mpox in the United States. The virus could potentially spread from DRC or other affected countries to the United States directly or through other countries via infected travelers. Broader regional spread of clade I mpox in DRC and neighboring countries increases the risk of importation to the U.S.

  • As of August 23, 2024, the DRC outbreak has resulted in more than 19,500 confirmed or suspected cases so far in 2024.
  • As of August 23, 2024, there are now new clade l mpox cases in non-endemic countries in Africa, including 190 confirmed cases in Burundi, 4 confirmed cases in Rwanda, 3 confirmed cases in Uganda, and 2 confirmed travel-associated case in Kenya.
    • Epidemiologic data are limited for patients with mpox, and we do not know the true number of cases, nor how each confirmed case acquired infection. However, Rwanda and Burundi both reported cases in people with no travel history to DRC, and sustained spread may be occurring in Burundi based on the geographic distribution of cases.
    • There are also outbreaks in countries neighboring DRC, including Republic of the Congo (ROC) (162 confirmed or suspected cases) and Central African Republic (CAR) (45 confirmed cases), although these countries are endemic for clade I mpox.
  • On August 15, 2024, a travel-associated clade I mpox case was confirmed in Sweden, the first clade I mpox case confirmed outside the African continent.
  • Evidence of sexual transmission and transboundary spread in Central and Eastern Africa could indicate an increased risk of spread if introduced to the United States. Recent genomic analysis suggests that transmission of clade Ib mpox in non-endemic countries and parts of DRC is primarily human-to-human, and not due to recurring zoonotic introductions. Nonetheless, we assess that widespread sexual transmission in the U.S. general population is unlikely.
    • The potential for sustained heterosexual transmission of clade I mpox is likely to be low in the United States based on lack of widespread transmission in heterosexual networks during the 2022 global clade IIb mpox outbreak and insights from modeling studies.
    • Modeling work from the United Kingdom related to the 2022 outbreak indicated that lower partnership formation rates among heterosexuals prevented sustained transmission within these networks. CDC modeling has shown similar results for heterosexual networks in the U.S.1
    • The likelihood of infection among gay, bisexual, and other men who have sex with men (MSM) is assessed separately in the risk assessment for MSM in the United States, reported below.

Even if clade I mpox reaches the United States, we assess that the United States is not likely to experience transmission levels similar to DRC and neighboring countries.

  • Experts believe transmission levels would likely be reduced in the United States for several reasons, including smaller average household sizes, increased access to improved sanitation and healthcare, and the lack of zoonotic reservoirs of disease.
  • CDC modeling suggests close-contact transmission within and between households is unlikely to result in a large number of clade l mpox cases in the United States.
    • Our model simulated household transmission through direct contact and transmission to other households through children's direct contact with others at daycares and schools. Direct contact involves skin-to-skin contact and other direct contact with virus, such as through sharing contaminated towels; this virus is not known to spread efficiently through respiratory secretions and is not spread via the air.
    • Modeling results are based on clade Ia dynamics and may be revised as new data become available from the clade Ib outbreak.

Impact

We assess the impact of infection for the general population as low to moderate. Factors that informed our assessment of impact included the following:

  • Most of the United States population has no immunity to mpox.
    • Acquired immunity from previous infection with the virus that causes mpox is extremely low for the general population (see separate analysis for MSM, who were disproportionately affected by mpox during the 2022 outbreak).
    • Vaccine-induced immunity from mpox vaccination during the 2022 outbreak is also very low across the general population. During the 2022 outbreak, vaccination was targeted to those at highest risk of infection, including MSM with certain risk factors and their partners.
    • Prior smallpox vaccination, which can offer some protection against mpox, has declined substantially in the United States since the 1970s, when population-wide smallpox vaccination ceased due to eradication of the disease.
  • Morbidity and mortality from clade I mpox are likely to be lower in the United States, compared to DRC and neighboring countries, because of the availability of high-quality supportive care and access to medical countermeasures.
    • DRC health authorities reported the case fatality rate (CFR) from suspected clade I mpox during 2024 as 3.04%; however, this may be an overestimation because of challenges in completeness of case testing and reporting. In a study of 216 patients with mpox from one DRC hospital from 2007-2011, investigators found a CFR of 1.4% among 216 patients who received limited supportive care and no mpox therapeutics. Another recent study indicated improved mortality rates (1.7%) among study enrollees who were offered supportive medical care.
    • In the United States, the CFR would likely be lower, given better access to high-quality supportive medical care and therapeutics.

Confidence

We have low confidence in this assessment.

We note uncertainty in the implications for the United States of the recent spread of the clade I mpox outbreak in Central and Eastern Africa and recent travel-associated cases. We have therefore changed our confidence in this assessment from moderate to low. We note other uncertainties around epidemiologic and genomic data in Central and Eastern Africa, including transmission dynamics in children and non-MSM sexual networks.

Risk assessment for MSM in the United States

Overall risk for this population

We assess the overall risk for gay, bisexual, and other men who have sex with men (MSM) in the United States who have more than one sexual partner, and their sexual partners regardless of gender, as low to moderate, with low confidence. Overall risk is assessed by considering the likelihood and impact of infection across the population (see Methods section). This assessment may change as new evidence becomes available.

Likelihood

We assess the likelihood of infection for MSM as low to moderate. Factors that informed our assessment of likelihood included the following:

  • There are no known cases of clade I mpox in the United States. However, the virus could potentially spread from DRC or neighboring countries to the United States or from other countries through infected travelers who expose others at their destinations.
    • Evidence of efficient sexual transmission and transboundary spread in Central and Eastern Africa could indicate an increased risk of spread if introduced to the United States.
  • Outbreaks of clade I mpox associated with sexual contact among MSM, and female sex workers and their contacts, have been reported in some DRC provinces. Health authorities documented sexually transmitted clade I mpox in DRC in MSM in six cases during 2023, indicating the virus could spread among these sexual networks in the United States if cases were imported.
  • During the ongoing 2022 global mpox outbreak, most U.S. cases were among MSM and their sexual partners, suggesting this population could be at increased risk for clade I mpox infection if the virus that causes clade I mpox were to spread to the United States. Furthermore, only a minority of MSM who CDC has recommended would benefit from vaccination have been fully vaccinated, though rates vary widely by jurisdiction.
  • There is some level of population immunity among MSM and their sexual partners following the 2022 outbreak of clade IIb mpox, which might reduce the risk of infection if clade I mpox were imported to the United States. In addition, during the 2022 global mpox outbreak in the U.S., MSM reported behavior adaptations that likely contributed to declines in mpox incidence, indicating that similar changes might be expected in response to future outbreaks.
    • A 2023 modeling study estimated that depending on population mpox immunity levels, jurisdictions face varying degrees of risk for sustained mpox recurrence, indicating many jurisdictions may have a high likelihood of sustained transmission, uncer certain epidemiologic scenarios.
    • More recent CDC modeling found that counties with higher population-level immunity had decreased chances of a prolonged or large outbreak. The study also found that if clade I mpox were to be introduced to MSM sexual networks in the United States, counties with greater than 50% population-level immunity would have smaller outbreaks on average (fewer than 100 infections).

Impact

We assess the impact of infection for this population as low to moderate. Factors that informed our assessment of impact included the following:

  • We expect the impacts of clade I mpox among MSM would likely be lower in the United States compared to DRC, because of the availability of high-quality supportive care and access to medical countermeasures.
  • Population immunity among MSM and their sexual partners is likely to additionally reduce the severity of infection.

Confidence

We have low confidence in this assessment.

We note uncertainty in the implications for the United States of the recent spread of the clade I mpox outbreak in Central and Eastern Africa and recent travel-associated cases. We have therefore changed our confidence in this assessment from moderate to low. We note other uncertainties around the level of prior immunity and the extent to which behavior adaptations initiated during the 2022 outbreak have continued or could recur.

Factors that could change our assessment

  • Detection of clade I mpox cases in the United States, particularly if there is domestic person-to-person transmission in jurisdictions with low estimated population immunity.
  • Detection of clade I mpox outbreaks and sustained transmission in Europe or other regions with high volume of travel to the United States.
  • Clade I mpox spreads extensively outside Central and Eastern Africa, including among people attending mass gatherings or among other highly mobile populations.

  • Further evidence of person-to-person spread among children in Central or East African countries, outside of already recognized high-risk activities, such as exposure to animals or close contact with persons with mpox.
  • Further evidence of widespread, prolonged chains of sexual transmission in non-endemic countries.

  • Further evidence of differentiated epidemiological patterns between mpox clades Ia and Ib.
  • Additional data to suggest an increased or decreased illness severity of clade I mpox infection.
  • Increased mpox vaccination coverage among high-risk groups in the United States.

Background and Methods

There is an ongoing outbreak of clade I mpox in DRC caused by the clade I Monkeypox virus, which is distinct from the clade IIb virus that caused the 2022 global outbreak. As of August 16, 2024, DRC reported 17,794 confirmed or suspected cases of mpox in 2024. In previous outbreaks, clade I has caused a higher number of persons to have severe disease and has been more transmissible than clade II within close-contact settings, typically in a household. Although clade I mpox is endemic in DRC, in 2023, health authorities reported a higher number of suspect cases and deaths across a wider geographic area that in some provinces affected atypical demographic groups. Approximately 66% of suspected mpox cases in DRC in 2024 were in children under age 15, similar to historical observations. However, adults were disproportionately affected in South Kivu province, where sexual transmission among adults was predominant.

Transmission in South Kivu—and, more recently, cases in several East African countries—has been associated with a newly identified sub-clade, designated as clade Ib. The age distribution of cases in Burundi—which has seen the most clade Ib cases outside of DRC—suggests that non-sexual transmission (likely through close physical contacts within households) of clade Ib is also occurring. Clade Ib is a more recently detected sub-clade, and data from the DRC and a previous animal study suggests decreased pathogenesis due to a genetic deletion in Ib, including lower case fatality rates in the DRC compared to clade Ia infections. However, additional data and studies are needed to make a definitive determination.

As of August 19, 2024, CDC has not detected any cases of clade I mpox in the United States, despite testing a high proportion of presumed mpox specimens—those positive for non-variola orthopoxvirus (NVO)—with tests that can identify mpox by clade. In addition, several commercial and other non-CDC laboratories perform clade II testing in addition to NVO tests, and all specimens tested to date have been clade II. If these laboratories see anything unusual—for example, an NVO-positive but clade II-negative result—they are recommended to alert CDC immediately to ensure additional genotyping is conducted to determine if they are clade I. CDC continues to work, including with other U.S. government agencies, on multiple approaches to expand clade-specific testing domestically. Several public health laboratories (PHLs) have begun or are working to begin clade-specific testing.

Spread of clade I mpox within DRC has likely resulted from transmission through several modes and in different settings. Most DRC cases in 2023 were in children, and in past outbreaks, children have been more likely to acquire infection—presumably through contact with infected animals or through limited person-to-person spread primarily in households. Transmission caused by close contact within households has occurred in past clade I mpox outbreaks. Historically, household transmission chains were typically small, although occasionally had involved up to six generations of transmission. While little data are available, household transmission is thought to be playing an important role in the current clade I outbreak in DRC, particularly for children. Transmission through patient care, in the absence of appropriate personal protective equipment, has also been documented. Transmission risk is highest among unvaccinated contacts; children and young adults are less likely to have vaccine-induced orthopoxvirus immunity since smallpox vaccination programs ended in 1980 in DRC. Sexual transmission has also been confirmed as an important route of transmission for the ongoing clade IIb outbreak in Eastern DRC, and likely plays an important role in regional transboundary spread; the South Kivu clade I mpox outbreak reportedly spread initially among female sex workers before becoming more broadly distributed. Sexual transmission of clade I mpox was also confirmed in a small outbreak affecting MSM and women in Kenge, Kwango Province, in March 2023. Recent studies indicate there is sexual transmission of clade I mpox with subsequent household spread occurring in parts of DRC and neighboring countries.

In 2023 and 2024, outbreaks were reported in urban areas of DRC, including Kinshasa and less-populated regions of DRC, such as Équateur and South Kivu Provinces, where cross-border movement elevates the risk of spread of the disease outside of the country. As of August 23, 2024, there are 162 clade I mpox cases (confirmed, probable, and suspected) in the neighboring Republic of the Congo and 45 clade I mpox cases (confirmed, probable, suspected) in the neighboring Central African Republic, where clade I mpox is endemic. As of August 23, 2024, there are 190 confirmed cases in Burundi, 4 confirmed cases in Rwanda, 3 confirmed cases in Uganda, and 2 confirmed cases in Kenya in transit passengers, representing the first cases of mpox reported in these countries. There are no direct commercial air passenger flights arriving from DRC and other neighboring countries into the United States. The recent confirmed clade I travel-associated cases outside of the African continent highlight the real risk for travel-associated importations. While not currently reported, if clade I mpox began to circulate person-to-person in other countries, the potential for importation to the United States would increase.

CDC subject-matter experts specializing in risk assessment methods, infectious disease modeling, global health, and mpox and other orthopoxviruses collaborated to develop this rapid assessment. Experts initially convened in February 2024 to discuss the need for an assessment examining the risks posed by the DRC outbreak to the United States, key evidence related to the DRC outbreak, and specific populations to include in the assessment. To conduct this assessment, experts considered evidence including epidemiologic data from Central and Eastern Africa, data from the ongoing mpox outbreak in the United States caused by clade IIb, and historical data on clade I mpox outbreaks in DRC. After the initial assessment was finalized in February, experts subsequently re-reviewed evidence and updated this assessment in early March, mid-March, mid-April, late June, and early August.

Overall risk was estimated by combining the likelihood of infection and the impact of the disease. For example, low likelihood of infection combined with high impact of disease would result in moderate risk. The likelihood of infection refers to the probability that members of the general U.S. population or MSM acquire mpox throughout 2024—which in turn depends on the likelihood of exposure, infectiousness of the disease, and susceptibility of the population. The impact of infection considers several factors affecting the consequences of infection, including the severity of disease, level of population immunity, availability of treatments, and necessary public health response resources. A degree of confidence was assigned to each level of the assessment, taking into account evidence quality, extent, and corroboration of information.

For more details on our methods, please see our rapid risk assessment methods webpage.

Previous Updates

See our previous Mpox Risk Assessment since July 2024.

See other archived previous updates of the Mpox Rapid Risk Assessment.

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Content Source:
Center for Forecasting and Outbreak Analytics
  1. We note that the general population includes gay, bisexual, and other men who have sex with men (MSM) who have more than one sexual partner, and their partners regardless of gender, but we assess this group separately because the clade I mpox outbreak in DRC and neighboring countries may pose a higher risk to this population.
  1. Pollock E, Nakazawa Y, Asher J, Gift T, Spicknall I. Potential mpox transmission among college-attending 18-25-year-olds with opposite-sex contacts in the United States. (2023, July 24-27). The International Society for Sexually Transmitted Diseases Research.