Vaccines Didn’t Turn Back Mpox, Study Finds. People Did.

Why It Matters: Vaccines often arrive too late to stamp out outbreaks.

Public health response to outbreaks often relies heavily on vaccines and treatments, but that underestimates the importance of other measures, said Miguel Paredes, lead author of the new study and an epidemiologist at the Fred Hutchinson Cancer Center in Seattle.

Although the Food and Drug Administration approved a vaccine for mpox in 2019, getting enough doses produced and into arms proved challenging for many months after the outbreak began. Vaccines for new pathogens are likely to take even longer.

The new analysis suggests an alternative. Alerting high-risk communities allowed individuals to alter their behavior, such as reducing the number of partners, and led to a sharp decrease in transmission, Mr. Paredes said. In North America, the outbreak began petering out in August 2022, when less than 8 percent of high-risk individuals had been vaccinated.

Public health messaging can “be really powerful to control epidemics, even as we’re waiting for things like vaccines to come,” he said.

The Long View: Vaccines are still important.

Some experts unrelated to the work were not convinced that behavioral change was largely responsible for stemming the outbreak.

“If the national numbers are driven by large outbreaks in a few places, then the folks at the highest risk in those places would get infected pretty quickly, and their immunity would be especially valuable in limiting the outbreak size,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

“Add in some vaccine-induced immunity in this group and a bit of behavior change, and it will be even more effective,” he said.

The Centers for Disease Control and Prevention worked closely with the L.G.B.T.Q. community to raise awareness about the importance of behavior modification, said Thomas Skinner, a spokesman for the agency.

While behavioral change can curtail outbreaks in the short term, vaccinations prevent the outbreak from resurging once people return to their normal routines, said Virginia Pitzer, an epidemiologist at the Yale School of Public Health.

“As we’ve seen with Covid, the behavioral change only lasts so long,” she said.

Back Story: The mpox outbreak was underway well before it was detected.

Mr. Paredes and his colleagues analyzed genetic sequences of the mpox virus from five global regions, along with air travel and epidemiological data. They were able to map the evolution of the virus to determine that the outbreak originated in Western Europe, most likely in Britain, some time between December 2021 and late March 2022. The first case was detected in Britain in May 2022.

In all five regions, the virus was spreading extensively long before it was detected by public health authorities. Later introductions from outside a particular region played a limited role in feeding the outbreak, accounting for less than 15 percent of new cases, the researchers said. That suggests that travel bans would have had only a minor impact.

The analysis also found that about one-third of infected individuals or less were responsible for most of the virus transmission as the outbreak waned.

“The most impact you can get from public health is not necessarily from these huge population-wide policies,” Mr. Paredes said. Instead, by focusing on this high-risk group, “you can go a really long way into controlling the epidemic.”

What’s Next: Surveillance prevents outbreaks.

The fact that the virus was circulating widely long before it was detected points to the need for better surveillance of pathogens — a lesson also learned from Covid, said Trevor Bedford, an evolutionary biologist at Fred Hutchinson Cancer Center, in whose lab Mr. Paredes works.

“If we can catch emerging pathogens earlier on, like even weeks, it will make a big difference in terms of changing the course of these epidemics,” Dr. Bedford said.

In the case of mpox, the pattern of virus spread was consistent with the volume of air travel between the United States and Western Europe.

“As soon as there was an outbreak of mpox in Western Europe, we should have known that we would be seeing cases in the U.S.,” Dr. Pitzer said.

The new study focused on the dynamics of the 2022 outbreak. But other research has shown that the mpox virus has been circulating among people since 2016.

“It remains a mystery to me how we could have sustained human-to-human transmission between 2016 and the beginning of 2022 and not have more of a visible epidemic,” Dr. Bedford said.