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Risk of Clade 1 Mpox Outbreaks Among Gay, Bisexual, and Other Men Who Have Sex With Men in the United States

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By Staff Middle Land

August 17, 2024
3:45 pm
Los Angeles, CA

By U.S. Centers for Disease Control and Prevention (CDC) To assess the impact among MSM of potential clade I mpox introduction into the U.S., CDC modeled varying levels of transmissibility and county-specific immunity. Modeling indicates as population-level immunity increased, the chances of a prolonged or large outbreak decreased. This report is one of two modeling studies exploring the potential spread of clade I mpox in the United States. This study explores the impacts of population-level immunity and transmissibility on sexual transmission of mpox among gay, bisexual, and other men who have sex with men. A separate study explores the impact of household and other non-sexual spread on number of cases and number of households infected in an outbreak. At a glance Clade I mpox is causing an ongoing outbreak in

By U.S. Centers for Disease Control and Prevention (CDC)

To assess the impact among MSM of potential clade I mpox introduction into the U.S., CDC modeled varying levels of transmissibility and county-specific immunity. Modeling indicates as population-level immunity increased, the chances of a prolonged or large outbreak decreased.

This report is one of two modeling studies exploring the potential spread of clade I mpox in the United States. This study explores the impacts of population-level immunity and transmissibility on sexual transmission of mpox among gay, bisexual, and other men who have sex with men. A separate study explores the impact of household and other non-sexual spread on number of cases and number of households infected in an outbreak.

At a glance
Clade I mpox is causing an ongoing outbreak in the Democratic Republic of the Congo (DRC). During the ongoing clade II mpox outbreak that began in 2022, the main transmission route was associated with sexual activity among certain gay, bisexual, and other men who have sex with men (MSM). As such, CDC modeled varying levels of transmissibility of clade I mpox and county-specific population-level immunity—from either previous infection or one or two doses of the JYNNEOS vaccine— among MSM in the United States to explore potential impacts of clade I mpox introduction to these sexual networks in the United States. Our modeling results indicate that:

Despite a higher presumed transmissibility of clade I mpox compared to clade II, in 13 U.S. counties, simulated clade I outbreaks among MSM were smaller than ongoing clade II outbreaks. The clade I outbreaks were likely smaller because of prior immunity from vaccination and previous infection, as well as the population’s related behavior change.
The size and duration of potential mpox outbreaks varied across individual counties due to differences in vaccination coverage and previous clade II mpox case rates, together referred to as population-level immunity.
As population-level immunity increased, the chances of a prolonged or large outbreak decreased. Therefore, CDC recommends all eligible people get vaccinated.

Background
In previous outbreaks in endemic African countries, clade I virus has caused a higher proportion of severe disease and been more transmissible than clade II within close-contact settings. To date, no cases of clade I mpox have been detected in the United States, despite wastewater monitoring for community detection and testing a high proportion of presumed mpox specimens with tests that can identify mpox by clade.

The current DRC outbreak of clade I mpox has likely resulted from transmission through several modes in different settings, including household, zoonotic, and sexual exposures. A 2024 outbreak in the Kamituga mining region in DRC has been associated with transactional sex, as 88% of hospitalized cases reported recent transactional sex. Additionally, health authorities reported six cases among MSM in a rural town in DRC in 2023. In a rapid risk assessment updated as of May 10, 2024, CDC assessed the risk posed by the clade I mpox outbreak in the DRC to the U.S. MSM population as low to moderate, higher than the risk posed to the general population.

Here, we use a dynamic, agent-based transmission model to explore in more detail the risk of prolonged sexual transmission among MSM in 13 counties in the United States if clade I monkeypox virus were to be introduced to this population. The 13 counties in this analysis were selected from 50 jurisdictions in the Ending the HIV Epidemic (EHE) Initiative to represent a range of population size and immunity profiles (see Methods for additional information). However, many of the EHE counties not included in our analysis likely are similar to the considered counties and would be expected to have similar probabilities of prolonged transmission.

Key findings
The county-level modeling results showing the size and duration of potential mpox outbreaks varied across individual counties due to differences in population-level immunity. For example, results indicated that outbreaks averaging more than 100 cases did not occur when population-level immunity from vaccination or previous infection was greater than 50% among MSM (Figure 1). Additionally, our simulation results showed that counties with higher population-level immunity had smaller outbreaks—defined as cumulative infections one year after introduction—and lower chance of prolonged transmission—defined as continued incident infections one year after introduction (Figure 2).

Our results suggest that high population-level immunity in a county, coupled with related behavior change, would lead to a much smaller mpox clade I outbreak among MSM compared to the ongoing mpox clade IIb outbreak. When comparing modeling results to number of confirmed diagnoses in the 2022 outbreak, only three of the 13 counties had more mpox diagnoses across all transmissibility scenarios modeled, and only two additional counties had more mpox diagnoses in the 20% more transmissible scenario modeled (data not shown to maintain anonymity). All five of these counties had estimated levels of population-level immunity below 25%.

Figure 1. Transmission of mpox among MSM in U.S. counties with a range of immunity profiles across three transmission scenarios: box plots of distribution of the cumulative number of infections expected within a year, following introduction of five individuals with mpox and with high levels of sexual activity. Transmission scenarios represent clade II (74.5% per-contact transmissibility), clade I (with 10% relative increased transmission over clade II), and clade I (with 20% relative increased transmission over clade II). (Photo:cdc.gov)

Figure 2. Transmission of mpox among MSM in U.S. counties with a range of immunity profiles across three transmission scenarios: probability of prolonged transmission after one year following introduction of five individuals with mpox with high levels of sexual activity. Transmission scenarios represent clade II, clade I (with 10% relative increased transmission over clade II), and clade I (with 20% relative increased transmission over clade II). (Photo:cdc.gov)

Among counties with population-level immunity lower than 50%, increases in immunity were correlated with lower probabilities of prolonged transmission among MSM for all three levels of transmissibility we analyzed (Figure 2). These results suggest that as the population-level immunity of a county increases, the probability of prolonged transmission decreases linearly, for all three levels of transmissibility we analyzed (Figure 2). In particular, counties with >50% population-level immunity had a <12% probability of prolonged transmission.

Public Health Considerations
Increasing population-level immunity through vaccination in counties can reduce both the number of infections as well as the chance of prolonged transmission of mpox. Vaccination is an important—yet underutilized—tool in preventing the spread of mpox. The Advisory Committee on Immunization Practices recommends people with potential risk of exposure to mpox receive two doses of the JYNNEOS vaccine. Despite this, only one in four of the approximately two million people recommended to receive the vaccine in the United States based on risk of exposure to clade II mpox have received both doses. JYNNEOS is also expected to protect against clade I mpox. CDC will continue to evaluate available data and assess the risk posed to MSM in the United States by the clade I mpox outbreak in DRC.