Monkeypox Outbreak Leaves Risks, Questions in Its Wake

“We can’t get lulled into this sense that monkeypox has disappeared,” said Jason Kindrachuk, an assistant professor at the University of Manitoba with a focus on emerging viruses.

Case numbers have been steadily declining since early August. Daily reported cases in the U.S. have fallen to around 40, from a peak of around 440. In Ontario, once a hot spot, health officials in the Canadian province said they are considering whether to declare the outbreak over.

The slowdown is attributed to a combination of a buildup of immunity and behavioral change, disease researchers said. The exact role each played hasn’t been determined. “They are working together in many cases,” said

David Heymann,

professor of infectious-disease epidemiology at the London School of Hygiene and Tropical Medicine.

Dozens of countries bet that Jynneos, a vaccine made by Denmark’s

Bavarian Nordic

A/S that had sat in stockpiles as a biodefense against a possible reintroduction of smallpox, could curb the spread of monkeypox, which is part of the same virus family. Studies on smallpox vaccines in Africa had found that they were around 85% effective at preventing monkeypox, but no such studies had been undertaken with Jynneos.

Early evidence from Jynneos’s use during the outbreak suggests the bet paid off. A recent study from the Centers for Disease Control and Prevention found that among men ages 18 to 49 in the U.S. who were eligible for Jynneos, case rates among the unvaccinated were 14 times higher than for those who had received at least one dose at least two weeks earlier. As of Oct. 18, around 647,400 people in the U.S. had received at least one dose of Jynneos, according to the CDC.

Immunity doesn’t fully explain the drop in cases, disease experts said. In the U.K., new cases started to fall before a vaccination campaign gained momentum, said Jake Dunning, senior researcher at the University of Oxford’s Pandemic Sciences Institute.

“Vaccine probably helped to bring things down and keep it as one curve, rather than more of a roller coaster,” he said.

Also driving down cases, disease experts said, was a reduction in sexual contact by men at the highest risk of catching monkeypox. In an August survey of around 800 men who have sex with men in the U.S., around half reported taking at least one measure in response to the monkeypox outbreak to limit their number of sexual contacts. Those measures included reducing one-time sexual encounters and cutting down the number of sex partners. A U.K. report published in September found that rates of two sexually transmitted diseases that also disproportionately affect men who have sex with men fell in August, suggesting that behavior change contributed to the decline in monkeypox.

Uncertainty regarding the precise roles played by immunity and behavior change mean that it is impossible to predict the trajectory of the virus, disease experts said. “If there’s a significant proportion that is attributed to behavior change, if that behavior change is not sustainable, will we see increases again?” said Anne Rimoin, professor of epidemiology at the University of California, Los Angeles, who has been researching monkeypox in the Democratic Republic of Congo for many years.

Even if the virus fades in some places, it is likely to be reintroduced through international travel because it is present in so many countries, said Emma Thomson, a professor of infectious disease at the University of Glasgow.

It hasn’t been determined whether the virus made its way into any new animal populations during the global outbreak. While monkeypox is mainly associated with forest-dwelling rodents in western and central Africa, it has been detected in other animals. An Italian greyhound in Paris caught monkeypox in June, likely from one of its owners, according to a case report in the Lancet.

“More human infections may arise because of that,” Geoffrey Smith, an expert on poxviruses at the University of Cambridge, said of potential animal reservoirs.

In 2003, around 50 people in the U.S. caught monkeypox from pet prairie dogs that had contracted the virus after sharing caging and bedding with small animals imported from western Africa. None of those cases went on to infect other people.

The global outbreak has prompted fresh calls for more research. A U.K. government-backed science funding group this week provided 2 million pounds, the equivalent of $2.2 million, for monkeypox research to 25 scientists spanning 12 universities. The researchers said their work would include detailed genomic sequencing, studies into the immune response to vaccination, developing new therapies and investigating the potential for animals to spread monkeypox.

Scientists said they want more research into monkeypox in central Africa, where a more-severe strain of the virus known as clade I circulates, to reduce transmission in countries there and to lower the risk of its sparking a more widespread outbreak. Dr. Dunning said that a global outbreak arising from the milder clade II virus raised the possibility that it could happen with clade I.

“That would be even more concerning,” he said.

Write to Denise Roland at denise.roland@wsj.com