Monkeypox Can Be Asymptomatic, Two Studies Suggest

European scientists have raised the alarm that a monkeypox (MPV) infection can be present in an asymptomatic patient. Evidence from two separate non-peer-reviewed studies suggests that not everybody who tests positive for the virus shows symptoms, raising the prospect the MPV outbreak can't be curbed by testing and quarantine alone.

Scientists found evidence that some people may be asymptomatic of MPV infection and therefore go undiagnosed. However, more research is necessary to determine whether asymptomatic people can spread MPV.

The research was published in the journalNature.

Researchers retrospectively analyzed 224 samples from men at a Belgian health clinic undergoing STI screening for gonorrhea and chlamydia in May 2022 to determine whether undiagnosed MPV infections occurred.

At the sampling time, three men had no symptoms, while one had a painful rash, the study states. After clinical examinations 21 to 37 days later, the three men denied experiencing any symptoms.

Each of the three patients had a blood test confirming MPV infection, and MPV was also cultured from two.

The study included men who have sex with men (MSM) living with HIV, MSM using PrEP, and MSM whose recent sexual partner was informed of gonorrhea or chlamydia infection.

The authors believe that testing and quarantining individuals reporting symptoms may not be sufficient to contain the current MPV outbreak.

Rather than simply identifying and isolating symptomatic individuals, the authors suggest reviewing specific measures and adjusting messaging to prevent human-to-human transmission.

"Firstly, awareness campaigns in the general and high-risk populations should include the possibility of asymptomatic transmission among close (sexual) contacts," the report states. "Secondly, efforts to identify asymptomatic cases should be increased by contact tracing and potentially by screening high-risk populations. Finally, our data provide additional evidence to introduce vaccination of high-risk populations."

In a second report published in Annals of Internal Medicine, researchers detected PCR results for the MPV virus in anal swab samples from asymptomatic men who had sex with men. It suggests vaccinations limited to people with known exposure to MPV may not be effective at preventing infection.

Bichat-Claude Bernard Hospital in Paris examined the risk of MPV spread via sex.

Anal swabs were taken from 200 men as part of a screening program for those taking HIV medications or PrEP.

A total of 13 samples, or 6.5 percent, tested positive for MPV. However, the report found that only two of the 13 patients who tested positive developed symptoms.

Although the men showed signs of the virus in their bodies, this study cannot determine whether they were contagious. The researchers indicated that ring postexposure vaccination --meaning vaccinating those around a positive-testing MPV patient -- may be insufficient in reducing the transmission.

Whether an asymptomatic patient can spread MPV remains to be learned. According to the researchers, "Whether this indicates viral shedding that can lead to transmission is unknown."

The Advocate reached out to representatives of the Department of Health and Human Services and the Centers for Disease Control and Prevention for the agencies' reaction to the research and will update this story when we hear back.