The findings, published in Annals of Internal Medicine, follow a similar, non–peer-reviewed report from Belgium. Researchers in both studies tested swabs for monkeypox in men who have sex with men. These swabs had been collected for routine STI screening.
It’s unclear whether asymptomatic individuals who test positive for monkeypox can spread the virus, the French team wrote. But if so, public health strategies to vaccinate those with known exposure “may not be sufficient to contain spread.”
In an editorial accompanying their paper, Stuart Isaacs, MD, associate professor at the University of Pennsylvania, Philadelphia, said it “raises the question of whether asymptomatic or subclinical infections are contributing to the current worldwide outbreak.”
Historically, transmission of monkeypox and its close relative, smallpox, was thought to be greatest when a rash was present, Dr. Isaacs wrote. “Long chains of human-to-human transmission were rare” with monkeypox.
That’s changed with the current outbreak, which was first detected in May. On Aug. 17, the World Health Organization reported more than 35,000 cases in 92 countries, with 12 deaths.
Research methods
For the French study, researchers conducted polymerase chain reaction tests on 200 anorectal swabs from asymptomatic individuals that had been collected from June 5 to July 11 in order to screen for gonorrhea and chlamydia. Of those, 13 (6.5%) were positive for monkeypox.
During the study period, STI testing had been suspended in individuals with monkeypox symptoms because of safety concerns, the researchers reported.
The research team contacted the 13 monkeypox-positive patients and advised them to limit sexual activity for 21 days following their test and notify recent sexual partners. None reported having developed symptoms, but two subsequently returned to the clinic with symptoms – one had an anal rash and the other a sore throat.
In the Belgian report, posted publicly on June 21 as a preprint, 3 of 224 anal samples collected for STI screening in May tested positive for monkeypox. All three of the men who tested positive said they did not have any symptoms in the weeks before and after the sample was taken.
At follow-up testing, 21-37 days after the initial samples were taken, all patients who had previously tested positive were negative. This was “likely as a consequence of spontaneous clearance of the infection,” the authors of that paper wrote.
Clinical implications of findings are uncertain
Monica Gandhi, MD, MPH, a professor of medicine at the University of California, San Francisco, said in an interview that the clinical implications of the findings are uncertain because it’s not known how much viral transmission results from asymptomatic individuals.
Dr. Monica Gandhi
Nevertheless, Dr. Gandhi said that “vaccinating all gay men for monkeypox who will accept the vaccine is prudent,” compared with a less aggressive strategy of only vaccinating those with known exposure, which is called ring vaccination. That way, “we can be assured to provide immunity to large swaths of the at-risk population.”
Dr. Gandhi said that movement toward mass vaccination of gay men is occurring in the United States, Canada, Europe, and Australia, despite limited vaccine supply.
She added that, although monkeypox has been concentrated in communities of men who have sex with men, “anyone with multiple sexual partners should be vaccinated given the data.”
However, a WHO official recently cautioned that reports of breakthrough infections in individuals who were vaccinated against monkeypox constitute a reminder that “vaccine is not a silver bullet.”