IE 11 is not supported. For an optimal experience visit our site on another browser.

People can be infected with monkeypox without showing symptoms, study suggests

Experts say it's possible that people who have the virus, but are asymptomatic might be able to infect others.
Doctor/Nurse with Swab Test Sample in Hospital, PCR Device
An analysis of previously collected anal swabs from men who had sex with men and had no monkeypox symptoms revealed that 6.5% were positive for the virus. Morsa Images / Getty Images stock

Some people infected with monkeypox may carry the disease — and possibly infect others — without ever experiencing symptoms, a new study suggests.

An analysis of previously collected anal swabs from men who had sex with men and had no monkeypox symptoms revealed that 6.5% were positive for the virus. The men, who were receiving therapies to protect them from developing AIDS or who were being treated for AIDS, were required to be tested for two sexually transmitted diseases every three months to continue receiving their treatment, according to the report published Monday in the Annals of Internal Medicine.

The new findings “tell us that some people may carry the virus in their anus without having any symptoms,” said study coauthor Dr. Jade Ghosn, a professor of infectious diseases at the Paris Cite University and head of the Sexual Health Clinic of the Bichat-Claude Bernard Hospital in Paris.

“Whether they might transmit the disease to someone else is not something we tested for,” Ghosn said. “While we can’t confirm whether there can be secondary transmission by asymptomatic people, most likely there can be. That would be in keeping with what we see in the clinic. We see a lot of people with monkeypox who had sex but their partners did not have any lesions in the anal area.”

To explore whether people could have asymptomatic monkeypox, Ghosn and his colleagues turned to swabs that had been collected earlier at their center as part of a screening program for gonorrhea and chlamydia in men had reported having multiple sexual partners and who were getting medication to prevent AIDS or to prevent infection with HIV.

The researchers examined only swabs from men who had tested negative for the two STDs, who did not show any lesions during exams by healthcare personnel and who had not reported any signs or symptoms of monkeypox. Of the 200 swabs that were testable for monkeypox, 13 were positive for the virus.

Among the 187 asymptomatic men who tested negative for monkeypox, three showed up in the clinic more than three weeks after their initial negative swab and reported monkeypox symptoms. When they were retested, they were positive for the virus.

There are two ways for people from groups that are at highest risk to protect themselves from getting the virus from asymptomatic people, Ghosn said. “First, they should get vaccinated as soon as possible according to local and national guidelines,” he added. “The only way to contain the epidemic is to have a significant amount of the population we are dealing with—men who have sex with men—vaccinated.”

“Until then the only way to protect yourself is to wear a condom during sex and to reduce the number of sexual partners,“ Ghosn said. “Otherwise, what will happen is the epidemic will spread out of this population and reach more women and children than we are seeing now.”

Ghosn notes that the tested samples came from June 5 through July 11 when the epidemic was just starting to gain steam. “In July and August there were skyrocketing numbers so we might see higher numbers in samples from then,” he added.

Future studies might also look for the presence of the virus in the throats of people who are asymptomatic, Ghosn said, adding that it’s possible that people who have the virus in their throats might infect others.

Thus far there have been only two deaths attributed to the monkeypox epidemic, one of which was in an immunocompromised patient, Ghosn said. Nevertheless, this is not a disease you want to catch.

“It’s painful and you have to be isolated for 21 days, which means you can’t go to work and you can’t see anybody,” Ghosn said. “If you’re living with someone either you have to move away or they have to move away. It’s very complicated having to deal with being isolated for 21 days.”

The new findings are “something I think every public health and infectious disease expert has been concerned about from the beginning,” said Dr. William Schaffner, a professor of infectious disease at the Vanderbilt University Medical Center in Nashville. Even when people develop lesions, “if they are cuddling with someone and the lights are out, the partner might not see them.”

What we don’t know yet, because there haven’t been studies, is whether the virus could be in exhaled breaths or in saliva, Schaffner said.

Dr. Richard Silvera was “not completely surprised.” Still, he said, the results of the study should be interpreted with caution. “Just because there is detectible virus doesn’t mean that it could infect another person,” said Silvera, an assistant professor of infectious diseases at the Icahn School of Medicine at Mount Sinai.

“The main takeaway should be that only vaccinating contacts of a case may not be the best to contain the virus,” Silvera said. “It confirms the strategy we are using here of vaccinating folks at the highest risk of catching monkeypox in the immediate future.”