Could Mpox Come Back? What a 2023 Model Said About 50 U.S. Cities and Counties

Since May 2022, the United States had diagnosed more than 30,000 mpox cases, mostly among gay, bisexual and other men who have sex with men (MSM). By the time of this 2023 report, diagnoses had fallen to about one a day. But vaccination coverage varied widely from place to place, so CDC asked: if mpox were reintroduced, where could it take off again — and how big could it get?

Visual abstract. CDC.

How the model works

CDC simulated sexual networks of MSM and introduced five infectious men with high levels of sexual activity. Depending on chance and the population's immunity, the virus either fizzled or started an outbreak — defined as transmission still going 3 months later.

• Immunity — from vaccine or past infection — was varied from 0% to 99%. One dose of JYNNEOS was assumed to cut susceptibility by 37%, two doses by 67%, and infection by 100%; higher vaccine estimates (65% and 83%) changed the results little.
• The model assumed no further vaccination after April 28, 2023, no change in behavior as cases appeared, and no loss of immunity over the two years modeled.
• It was applied to the 50 local jurisdictions of the federal Ending the HIV Epidemic initiative — many of them large cities — using each one's vaccination and case data.

Every point of immunity counts

The risk of a recurrent outbreak, if mpox returned, fell in a straight line as immunity rose: each percentage point of immunity cut the risk by 0.62 points. There was no threshold below which a place was safe — more coverage always helped.

The size of an outbreak behaved differently, with thresholds:

Predicted cumulative infections in a recurrence, relative to the 2022 outbreak, by immunity level. CDC.

Where the risk was

15 of the 50 jurisdictions had enough immunity to be at minimal risk. They held 44% of the at-risk MSM in these areas — including Los Angeles, New York City, San Francisco and Washington, D.C. — which left 56% living where a reintroduction could spark sustained spread.

Places that reached high coverage were often those hit hardest early, which got vaccine first while demand was high; elsewhere, vaccine arrived after the outbreak had peaked and people felt less at risk.

Limits

The model assumed mpox would be reintroduced; a lower chance of reintroduction lowers the risk in proportion. Immunity will fade as the population turns over. Small outbreaks can happen even where the risk of a large one is low — Chicago reported a new cluster in April 2023. Much of the behavior data came from 2012 surveys.

What CDC recommended

Keep mpox vaccination accessible and sustained; vaccinate before Pride and other events, when interest is high, rather than after the virus returns; and focus on low-coverage areas and, everywhere, on younger and newly sexually active MSM and groups with low coverage. CDC recommends the full two-dose JYNNEOS course for MSM and others at risk.

Sources

Based on Pollock ED, Clay PA, Keen A, et al., "Potential for Recurrent Mpox Outbreaks Among Gay, Bisexual, and Other Men Who Have Sex with Men — United States, 2023," MMWR Morbidity and Mortality Weekly Report volume 72, number 21, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The imported copy was cut short; the missing text is from the report's full text in PubMed Central (PMC10231941).