A Kaiser Permanente study of nearly 8,000 men shows that in mid- to late 2024, mpox was far more common among men who have sex with men than previously thought. Individuals without symptoms accounted for most infections and likely played a prominent role in transmission, contrary to prior assumptions that people had to be symptomatic to spread the disease.
The study was published in Nature Communications.
“We have not known how Mpox is transmitted, and why the cases seem to have very few connections to other cases,” said the study’s senior author, Sara Y. Tartof, PhD, MPH, a scientist with the Kaiser Permanente Southern California Department of Research & Evaluation. “However, these findings help resolve a fundamental question in the epidemiology of Mpox by suggesting that infected people pose a risk of transmitting the disease to others even in the absence of clinical symptoms.”
In 2022, there was a global outbreak of mpox (previously referred to as monkeypox), which included the United States. Despite a vaccine, the disease has continued to spread, mostly among men who have sex with men. Classic symptoms of the disease include fever, chills, muscle pain, and swollen lymph nodes followed by a painful rash. The Centers for Disease Control & Prevention (CDC) has advised that it is people with symptoms who drive the spread of the disease. Yet a majority of cases lack any known connection to a symptomatic partner.
To resolve the mystery, researchers from Kaiser Permanente in collaboration with UC Berkeley School of Public Health, tested for the mpox virus using rectal swabs taken to check for other sexually transmitted diseases from men who have sex with men in Southern California during the summer and early fall of 2024. At the same time, the researchers watched electronic health records to see new mpox diagnoses among men enrolled in Kaiser Permanente Southern California health plans.
“We used the specimens from routine testing for other sexually transmitted diseases to test for mpox and found roughly 1% of men had asymptomatic infections without knowing it,” said the study’s first author, Joseph A. Lewnard, PhD, an associate professor of epidemiology at the UC Berkeley School of Public Health. “From the testing, we estimated that only about one in every 33 infections gets diagnosed and confirmed this by analyzing transmission patterns revealed by viral genomic sequences.”
These genomic analyses also allowed researchers to examine whether undiagnosed infections played a role in transmission.
“From the genomic data, we can reconstruct patterns of transmission looking backward in time,” explained Dr. Tartof. “These patterns were inconsistent with a scenario in which the 3% of infections who received diagnoses cause all onward spread.”
The study also showed that the mpox vaccine reduced the risk of being diagnosed with mpox by 78 percent and reduced the risk of infection by 50 percent. The study authors said that a renewed focus on mpox vaccination could play a significant role in controlling the outbreak.
“There was a big push to vaccinate people who could be at risk when the outbreak started in 2022,” Dr. Lewnard said. “However, vaccination rates have since decreased, and certain populations may not have been reached during the initial campaign, such as people who were in long-term relationships or who were younger and not sexually active in 2022.”
He added: “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.”
