Study Finds Hidden Mpox Infections May Drive Ongoing Spread

The role of people without symptoms has remained uncertain, limiting clarity on how transmission persists. Defining this pathway is important for directing testing within sexual health services. New reserach now indicates that asymptomatic, undiagnosed infections are common and likely contributed substantially to spread.

Kaiser Permanente led an analysis that leveraged specimens and data already generated in clinical care to characterize silent transmission. Investigators tested rectal swabs—originally collected for routine sexually transmitted disease screening—for mpox, monitored electronic health records for new diagnoses among health plan members, and applied viral genomic sequencing to reconstruct transmission patterns. This combined screening, surveillance, and phylogenetic approach was designed to capture infections that standard symptom-driven case finding would miss.

The study evaluated nearly 8,000 men who have sex with men (MSM) in Southern California during the summer and early fall of 2024. While classic symptoms of mpox include fever, chills, myalgias, lymphadenopathy, and a painful rash, the cohort strategy focused on detecting infections irrespective of clinical presentation. Concurrent monitoring of electronic health records among Kaiser Permanente Southern California enrollees enabled linkage of undiagnosed infections to subsequent case ascertainment.

Testing of repurposed rectal swabs identified asymptomatic infections in approximately 1% of MSM who were unaware of their status. Modeling based on these data indicated that only about one in 33 infections was diagnosed during the study window. Genomic analyses showed transmission patterns inconsistent with a scenario in which the small fraction of diagnosed cases accounted for all onward spread, underscoring the contribution of undetected infections. The study also found that vaccination reduced the risk of being diagnosed with mpox by 78% and reduced the overall risk of infection by 50%.

The work was conducted by Kaiser Permanente in collaboration with the University of California, Berkeley School of Public Health and is published in Nature Communications. The authors noted that increasing vaccination coverage could play a significant role in controlling ongoing transmission.

“We have not known how mpox is transmitted, and why the cases seem to have very few connections to other cases. 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,” said Sara Y. Tartof, scientist with the Kaiser Permanente Southern California Department of Research & Evaluation.

"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. 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,” said Joseph A. Lewnard, associate professor of epidemiology at the UC Berkeley School of Public Health.

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