As the number of confirmed monkeypox cases in the United States climbs, the need to contain the virus is clear. But what will it take to bring the outbreak under control?
A new study from the Yale School of Public Health (YSPH) sets specific targets for containment and offers multiple strategies for limiting the spread of monkeypox using basic public-health tools of community detection, contact tracing, and vaccination.
The study, currently in preprint, was led by Gregg Gonsalves, an associate professor in the Department of Epidemiology (Microbial Diseases) and a member of the school’s highly respected modeling unit.
“The goal is containment. But no one had really said what we need to do to get to containment,” said Melanie Chitwood, the study’s lead author, and a YSPH doctoral student. “What we wanted to do was put a quantitative framework around that goal.”
Braking an Outbreak
The research team used mathematical modeling to calculate how much testing, contact tracing, and vaccination of high-risk men who have sex with men (MSM) would be needed to suppress the spread of monkeypox. Current infection rates show that high-risk MSM are especially vulnerable to monkeypox, which is primarily spread through close physical contact and contact with infected items such as bedding, towels, and clothing,
Estimating that there are about 498,000 high-risk MSM in the U.S., the team found several paths to containment. Each option arrives at roughly the same result:
Option 1: If officials detect at least 40% of cases through testing and trace at least half of each patient’s close contacts, each of whom then quarantines, then vaccination is unnecessary.
Option 2: If testing finds only 10% of cases and no contact tracing takes place, then vaccination would need to reach between 12% and 47% of high-risk MSM.
Option 3: If testing detects at least 20% of cases and at least 25% of contacts are traced and quarantined, then vaccination would need to reach between 5% and 43% of high-risk MSM.
The third scenario translates into giving 49,800 – 428,080 doses of the Jynneos monkeypox vaccine to 24,900 – 214,040 high-risk MSM. (Full vaccination with Jynneos requires two doses, each given four weeks apart.)
The vaccination scenarios have ranges that depend on how many people each infected person subsequently infects.
If “secondary infections” are less likely, fewer vaccinations are necessary. But the more easily the virus spreads—and the less effective detection and contact tracing are—the higher vaccine coverage would need to be.
The authors concluded that vaccinating at least one-third of high-risk MSM, which translates into at least 329,000 doses, would support containment efforts in most scenarios.
“The strongest lever here is to get people vaccinated, and to do it quickly,” Chitwood said.
The power—and the limits—of mathematical modeling
Modeling allows researchers to examine questions that can’t easily be tested with real-world studies and to identify the potential implications of different events, interventions, and decisions.
“Modeling is a very clear way to see how the world works,” Chitwood explained. “It asks ‘What if?’ kinds of questions.”
Model results aren’t meant to be the final word. Rather, they create a framework for policymakers seeking to optimize public health interventions and outcomes. Indeed, the YSPH team behind the monkeypox model was invited to share the results with the CDC’s Center for Forecasting and Outbreak Analytics.
The current study has several important limitations. To simplify the calculations, Gonsalves’ team made a number of assumptions, including assuming the virus will not spread much outside the high-risk MSM population. If it does, the researchers warned, vaccination levels may need to be higher. They also assumed vaccines would be fully protective against infection, that they would reach the men who need them, and that only symptomatic men spread the infection.
For reasons like these, Chitwood said, the study’s estimates should be considered to conservative. “When we talk about the number of doses we need, we're assuming this dream scenario where we know exactly who our high-risk individuals are, we're only giving the doses to them, and we're not wasting any doses,” she said. “We know that’s very difficult.”
Gonsalves and A. David Paltiel, a professor in the Department of Public Health (Health Policy), are now using mathematical models to study how monkeypox may spread on university campuses.
As of August 25, there were 16,603 confirmed cases of monkeypox in the U.S., according to the CDC.
Despite a sustained increase in cases since March, Chitwood is confident the monkeypox outbreak can be brought under control.
“This is a very different bug than SARS-CoV-2,” she said. “What we do in the next weeks to months in terms of vaccine availability, testing availability, TPOXX [medication] availability—all of those things are going to make a big difference.”
Gonsalves was principal investigator on the study; Jiye Kwon, Alexandra Savinkina, and Jo Walker are co-authors; all are of the Yale School of Public Health. Co-author Alyssa Bilinski is affiliated with the Brown School of Public Health.