infected people pose a risk even in the absence of clinical symptoms
A large study of nearly 8,000 men in Southern California has quietly rewritten the understood story of how mpox spreads — revealing that the virus moves most freely not through the visibly ill, but through the silently infected. Researchers at Kaiser Permanente found that only one in thirty-three mpox infections is ever diagnosed, suggesting the true scale of the outbreak dwarfs what official counts have captured. The discovery challenges years of public health guidance and raises a harder question: how do you contain a disease that rarely announces itself?
- The virus has been spreading largely undetected — roughly 99% of infections go undiagnosed, meaning official case counts represent only a fraction of the true outbreak.
- Genomic tracing of transmission chains contradicts CDC guidance, showing that asymptomatic carriers — not symptomatic patients — are driving the spread.
- Vaccination rates have fallen since the 2022 push, leaving younger people, those in long-term relationships, and other underreached groups exposed to a risk far greater than they knew.
- The mpox vaccine offers meaningful but incomplete protection — 78% against diagnosed illness and 50% against infection — making renewed campaigns urgent but not sufficient alone.
- Public health systems built to track visible disease are structurally blind to this outbreak, demanding a fundamental shift toward proactive screening rather than reactive response.
In the summer and fall of 2024, researchers at Kaiser Permanente made a discovery that quietly overturned what public health officials believed about mpox. Testing nearly 8,000 men who have sex with men in Southern California — using rectal swabs originally collected for routine STI screening — they found that roughly one in every hundred men carried the mpox virus without any symptoms at all. When they traced the genetic fingerprints of the virus backward through time, the picture grew more unsettling: the vast majority of new infections could not be linked to anyone who had felt sick. The disease was spreading in silence.
This contradicted years of official messaging. The CDC had long held that symptomatic cases — the fever, rash, and characteristic lesions — were the primary engine of transmission. But the genomic data told a different story. "These findings help resolve a fundamental question in the epidemiology of mpox," said senior author Sara Tartof, "by suggesting that infected people pose a risk of transmitting the disease even in the absence of clinical symptoms."
The scale of hidden infection was stark. Researchers estimated that only about one in thirty-three infections was ever diagnosed and reported — meaning the true burden of disease was roughly thirty times larger than official counts suggested. The study, published in Nature Communications in collaboration with UC Berkeley, used electronic health records alongside the screening data to measure both the visible and invisible dimensions of the outbreak simultaneously.
The findings carried direct implications for vaccination. The mpox vaccine reduced the risk of a confirmed diagnosis by 78% and the risk of infection itself by 50% — real protection, but not absolute. Yet vaccination rates had declined since 2022, and entire populations had never been effectively reached. "Unvaccinated people face risk of severe disease," noted first author Joseph Lewnard, "and our findings suggest this risk is greater than we previously understood."
What the study ultimately revealed was a disease moving through communities in ways that traditional surveillance was never designed to detect. Asymptomatic carriers were not rare exceptions — they were the norm. Containing the outbreak would require not just treating those who felt ill, but finding and vaccinating people who had no idea they were at risk at all.
In the summer and fall of 2024, researchers at Kaiser Permanente made a discovery that upended what public health officials thought they knew about mpox transmission. By testing nearly 8,000 men who have sex with men in Southern California—using rectal swabs originally collected for routine screening of other sexually transmitted infections—they found that roughly one in every hundred men carried the mpox virus without showing any symptoms at all. More striking still: when they traced the genetic fingerprints of the virus backward through time, they found that the vast majority of new infections could not be explained by contact with people who actually felt sick. The disease, it turned out, was spreading silently.
This finding contradicted years of public health messaging. The Centers for Disease Control and Prevention had consistently advised that symptomatic cases drove mpox transmission—that the fever, chills, muscle pain, and characteristic rash were the markers of contagiousness. Yet when researchers analyzed the actual transmission patterns encoded in viral genomes, they discovered something different: undiagnosed, asymptomatic infections were doing much of the work. "We have not known how mpox is transmitted, and why the cases seem to have very few connections to other cases," said Sara Tartof, the study's senior author and a scientist with Kaiser Permanente's 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."
The scale of hidden infection was sobering. When researchers estimated how many cases were actually occurring versus how many were being diagnosed, they found that only about one in every thirty-three infections was ever confirmed and reported. The math was stark: if roughly one percent of men in the study carried the virus without knowing it, and only three percent of all infections resulted in a diagnosis, then the true burden of disease was roughly thirty times larger than official case counts suggested. A global outbreak that had begun in 2022 and continued to spread primarily among men who have sex with men was far more extensive than surveillance systems had captured.
The study, published in Nature Communications, was conducted in collaboration with UC Berkeley's School of Public Health. Researchers examined electronic health records alongside their testing data, watching for new mpox diagnoses while simultaneously screening for the virus in specimens that had been collected for other purposes. This dual approach allowed them to see both the diagnosed cases and the invisible ones—and to measure the gap between them. "From the testing, we estimated that only about one in every 33 infections gets diagnosed," explained Joseph Lewnard, the study's first author and an associate professor of epidemiology at UC Berkeley. "From the genomic data, we can reconstruct patterns of transmission looking backward in time. These patterns were inconsistent with a scenario in which the three percent of infections who received diagnoses cause all onward spread."
The findings had immediate implications for vaccination strategy. The study showed that the mpox vaccine reduced the risk of diagnosis by seventy-eight percent and reduced the risk of infection itself by fifty percent—meaningful protection, but not absolute. Yet vaccination rates had declined since the initial push in 2022, and certain populations had never been reached effectively: people in long-term relationships, younger individuals who were not sexually active when the outbreak began, and others on the margins of public health campaigns. Lewnard noted the consequence plainly: "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."
What the Kaiser Permanente study revealed was a disease moving through populations in ways that traditional surveillance could not detect. Asymptomatic carriers were not rare exceptions but rather the norm—the majority of infections, in fact. They were spreading the virus to others who might or might not develop symptoms, who might or might not seek testing, who might or might not be counted in official statistics. The outbreak was larger, more diffuse, and more difficult to interrupt than anyone had realized. The path forward required not just treating symptomatic cases but finding and vaccinating people who did not know they were at risk—a far more complex public health challenge than responding to visible disease.
Citas Notables
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.— Sara Tartof, Kaiser Permanente Department of Research & Evaluation
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.— Joseph Lewnard, UC Berkeley School of Public Health
La Conversación del Hearth Otra perspectiva de la historia
So the study found asymptomatic infections. But how do we know they're actually transmitting the virus if the people don't have symptoms?
That's where the genomic analysis comes in. Researchers can sequence the virus from diagnosed cases and trace it backward, like following a family tree. If the virus sequences don't match up with transmission from symptomatic people, then something else is spreading it.
And that something else is people who don't know they're infected.
Exactly. The genetic patterns showed that undiagnosed infections had to be driving spread. You can't explain the diversity and distribution of cases any other way.
Why weren't these asymptomatic cases being caught before?
Because nobody was looking for them. Testing was reactive—people got symptoms, sought care, got tested. This study used routine STI screening as a backdoor way to find infections that would otherwise have gone undetected.
So the real outbreak is thirty times bigger than we thought.
At least. One in thirty-three infections gets diagnosed. That means the disease is far more embedded in these communities than case counts suggest, and vaccination becomes even more urgent.
But the vaccine isn't perfect.
No. It cuts diagnosis risk by seventy-eight percent and infection risk by fifty percent. That's good, but it means vaccinated people can still get infected and potentially spread it. The real problem is that vaccination rates have dropped since 2022, and some populations were never reached in the first place.