Background
A major Kaiser Permanente study of nearly 8,000 men has found that mpox was far more common than previously understood during mid-to-late 2024, with individuals showing no symptoms accounting for the majority of infections and likely playing a central role in ongoing transmission.
The research, published May 14, 2026 in the journal Nature Communications, challenges the long-standing assumption that people must be symptomatic to spread the disease — a belief that has underpinned public health guidance from the Centers for Disease Control and Prevention (CDC).
“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, Dr Sara Y. Tartof, 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.”
**How the Study Was Conducted**
Researchers from Kaiser Permanente, collaborating with the University of California Berkeley School of Public Health, tested for the mpox virus using rectal swabs taken during routine sexually transmitted infection screening of men who have sex with men in Southern California during the summer and early autumn of 2024. Simultaneously, they monitored electronic health records for new mpox diagnoses among men enrolled in Kaiser Permanente Southern California health plans.
Key Developments
“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, Dr Joseph A. Lewnard, an associate professor of epidemiology at UC Berkeley. “From the testing, we estimated that only about one in every 33 infections gets diagnosed and confirmed — this by analysing transmission patterns revealed by viral genomic sequences.”
**Genomic Evidence of Silent Spread**
To determine whether undiagnosed infections contributed to transmission chains, the research team examined viral genomic data. “From the genomic data, we can reconstruct patterns of transmission looking backward in time,” Dr Tartof explained. “These patterns were inconsistent with a scenario in which the 3% of infections who received diagnoses cause all onwards spread.”
The findings carry significant implications for outbreak control strategies. Despite a global mpox outbreak beginning in 2022 and the availability of a vaccine, the disease has continued to circulate, predominantly among men who have sex with men. Classic symptoms include fever, chills, muscle pain, swollen lymph nodes and a painful rash — yet a majority of documented cases lacked any known connection to a symptomatic partner, a pattern this study now explains.
**Vaccine Effectiveness Confirmed**
Analysis
The study also provided robust confirmation of vaccine efficacy. The mpox vaccine reduced the risk of receiving a diagnosed mpox infection by 78% and reduced the risk of any infection — symptomatic or not — by 50%.
“Vaccination rates have since decreased, and certain populations may not have been reached during the initial 2022 campaign,” Dr Lewnard noted. “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.”
**Implications for Public Health**
The researchers are calling for a renewed public health focus on mpox vaccination, particularly in communities most at risk. The findings also underscore the need for surveillance strategies that go beyond symptom-based case detection — a critical lesson for controlling future outbreaks of sexually transmitted infections where asymptomatic transmission is suspected.
The study was published as DOI: 10.1038/s41467-026-72749-2 in Nature Communications on May 14, 2026.
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Keywords: mpox, monkeypox, asymptomatic transmission, sexually transmitted infections, Kaiser Permanente, UC Berkeley, Nature Communications, vaccination, public health
