Since May 2022, the United States has reported more than 30,000 cases of mpox, largely affecting gay, bisexual, and other men who have sex with men (MSM). Behavioral adaptations and vaccination helped slow the outbreak, and vaccine demand has decreased with falling mpox diagnoses. Without additional vaccination, population-level immunity will gradually decline. Here we estimate the likelihood and magnitude of resurgent mpox outbreaks in the future.
Methods
We used a dynamic network model of sexual partnerships among MSM, where individuals may have zero, one or two doses of the JYNNEOS mpox vaccine or have infection-induced immunity, based on a preprint estimating the 2022 mpox dynamics in DC plus additional vaccination uptake data through February 2023. We modeled this population forward in time while allowing MSM to age out, being replaced by younger, fully susceptible MSM. For each year, we recorded population-level immunity, the probability of an outbreak resurgence that lasted longer than three months, and the potential magnitude of the resurgence over two years if five infectious cases entered the population.
Findings
In early 2023, with 59% of the population having some immunity to mpox, we estimated a 4.6% (95% CI: 3.4%–5.9%) probability of an outbreak with 72 median cumulative infections (IQR: 56–98), conditional on reintroduction. Population-level immunity had a linear effect on resurgent outbreak probability. In contrast, immunity had a threshold effect on resurgent outbreak magnitude. Immunity declining from 59% to 30% caused small increases in magnitude; declining from 30% to 18% caused slightly larger increases; and when immunity declined below 18% there very were large increases, with more than 3000 infections and epidemics persisting after one year.
Conclusions
Continued vaccination among those at risk of future mpox exposure would lower the likelihood and size of future outbreaks. While immunity in DC is currently high, many other areas may be below the threshold expected for large outbreaks. Understanding immunity in key localities and the social and sexual engagements among MSM with and without immunity would improve resurgent outbreak estimates. Future vaccination efforts could also include outreach to newly sexually active or newly at risk MSM.
