A new study published in Nature Communications challenges the assumption that only people with symptoms can spread mpox. The study found that many men who have sex with men (MSM) tested positive for mpox even though they had no symptoms. Researchers then confirmed that these men likely passed on mpox to partners without ever knowing they had the virus.

Mpox is a virus that causes flu-like symptoms and a rash or sores on the skin. The sores sometimes appear on the genitals and may be mistaken for herpes or syphilis. Symptoms can last two to four weeks but most infections resolve without treatment or hospitalization.

Mpox is passed on through skin-to-skin contact or contact with an infected person’s clothing, towels, or bedding. While not technically a sexually transmitted infection (STI), mpox is passed through close physical contact. It is often spread through sexual behavior.

A global outbreak of mpox began in May 2022. It affected some groups more than others, especially gay and bisexual men, other men who have sex with men, and transgender people.

Most experts believed that mpox was primarily spread by people who had active symptoms like sores. However, contact tracers who tried to track outbreaks from one diagnosed patient to another had trouble finding connections between cases. This new study may explain why.

Study Shows that Many with Mpox Have No Symptoms

The study included almost 8,000 MSM who got routine STI testing at Kaiser Permanente in Los Angeles. Researchers used the samples collected to look for mpox. They found that roughly 1% of the men tested positive despite never having had symptoms. Based on this, they estimated that only one in every 33 mpox infections get diagnosed.

Researchers then looked at the genetic makeup of each positive sample to better understand how mpox is spread. The patterns confirmed that the virus was not just being spread by those who had symptoms and been diagnosed.

The study also found that the mpox vaccine, called JYNNEOS, reduced the risk of infection by 50 percent. The vaccine is given in two doses, four weeks apart. The Centers for Disease Control and Prevention (CDC) recommend JYNNEOS  for people who are likely to be exposed to mpox, including gay, bisexual, and other men who have sex with men/transgender partners who have engaged in certain behaviors.

Specifically, the agency suggests the vaccine for members of this community who, in the last six months:

• have had more than one partner
• been diagnosed with an STI
• visited commercial sex venues (like sex clubs or parties)
• had sex at a large public event (like a festival) in areas where mpox transmission is occurring.

Partners of people who fit these criteria should also get vaccinated. The vaccine is also recommended for anyone who has HIV or other immune system issues.

During the global outbreak the CDC and health departments around the country made a big push to get people vaccinated. But vaccination rates have been dropping since. The researchers involved in this new study believe that we need new efforts to increase vaccination rates. Joseph A. Lewnard, PhD, who is an associate professor of epidemiology at the UC Berkeley School of Public Health and the lead author of this study, said, “Unvaccinated people face risk of severe disease if they are exposed to mpox, and our findings suggest this risk is greater than we previously understood.”