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Mpox Studies Reveal Asymptomatic Spread, Long-Term Effects, and Emergence of Severe Strain

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New Mpox Research Reveals Asymptomatic Spread, Long-Term Effects, and Severe Strain

A series of recent studies and public health reports have provided new data on mpox (formerly known as monkeypox), covering asymptomatic transmission, long-term physical effects, and the identification of a more severe viral strain in the United States.

Asymptomatic Infections and Undiagnosed Cases

A study published on May 13, 2026, in Nature Communications by Kaiser Permanente and the University of California, Berkeley, analyzed nearly 8,000 men. The research found that in mid- to late 2024, mpox was more common among men who have sex with men than previously estimated.

Key findings from the research:

  • Researchers tested rectal swabs collected for other sexually transmitted infection screenings. Approximately 1% of men tested positive for mpox without reporting symptoms.
  • The study estimated that only about one in every 33 mpox infections was diagnosed and confirmed.
  • Genomic analysis indicated transmission patterns that could not be fully explained by diagnosed cases alone, suggesting that individuals without symptoms played a role in onward transmission.

Vaccine Effectiveness

The study found that the mpox vaccine reduced the risk of being diagnosed with mpox by 78% and the risk of infection (including asymptomatic cases) by 50%.

Statements from Researchers

Sara Y. Tartof, PhD, MPH, study senior author and scientist at Kaiser Permanente Southern California, stated the findings help address a fundamental question in mpox epidemiology by suggesting infected people can transmit the virus without clinical symptoms.

Joseph A. Lewnard, PhD, study first author and associate professor of epidemiology at UC Berkeley, stated that testing suggests only a fraction of infections are diagnosed. He also noted that unvaccinated individuals face a risk of severe disease that may be greater than previously understood.

The study was funded by the Outbreak Analytics and Disease Modeling Network, a CDC initiative.

Long-Term Physical Effects

A separate study conducted by researchers from the CDC, Columbia University, and the University of Texas Health Science Center examined more than 300 adults diagnosed with mpox during the 2022 outbreak. The findings indicated that 58% of those diagnosed between May 2022 and January 2023 continued to experience physical effects 11 to 18 months later.

Reported Persistent Issues

  • Lesions in one or more areas of the body
  • Difficulty defecating or stool incontinence
  • Urinary hesitancy or incontinence
  • Reduced range of motion
  • Neurologic impairment
  • Fatigue

Correlation with Socioeconomic Factors

The study reported a link between ongoing physical effects and socioeconomic factors. Patients experiencing unstable housing, unemployment, disability, or food insecurity were more likely to report that mpox-related effects impacted their employment.

Background on 2022 Outbreak

  • The U.S. outbreak began in May 2022, leading to over 30,000 cases.
  • In August 2022, the CDC and the World Health Organization declared the outbreak a public health emergency.
  • The majority of cases were reported among men who have sex with men.
  • Daily case numbers decreased significantly by late fall 2022, attributed to behavioral changes and a vaccination campaign.

First Severe Clade I Case Identified in New York City

The New York City Health Department has identified the first case of severe mpox caused by the clade I strain in the city.

  • One resident tested positive for clade I, a genetic group associated with more severe symptoms. This differs from clade II, which was responsible for the 2022 global outbreak and has a survival rate exceeding 99.9%.
  • Clade I was linked to a 2024 outbreak in the Democratic Republic of Congo and other regions in Africa.
  • The patient recently traveled internationally. No local transmission of mpox clade I has been identified in New York City, and the risk to local residents is currently considered low.

Transmission and Prevention

  • Mpox clade I spreads through direct contact, including sexual contact, kissing, and close respiratory droplets. It does not spread over longer distances through the air.
  • The NYC Health Department recommends at-risk individuals receive the two-dose mpox vaccine series.
  • The CDC advises vaccination for gay, bisexual, and other men who have sex with men aged 18 or older who have specific risk factors, as well as individuals planning to travel to areas where clade I is circulating.
  • Individuals who may have had close contact with an infected person should get vaccinated within 14 days.
  • Immunocompromised individuals and very young children face the highest risk of severe infection.
  • The JYNNEOS vaccine is typically used. Individuals who have previously had mpox do not require vaccination.

Symptoms and Treatment

Symptoms typically emerge one to three weeks after exposure and include a rash (which can be painful and blistery), fever, chills, exhaustion, muscle aches, headache, swollen lymph nodes, and respiratory symptoms.

  • In rare instances, mpox can lead to complications such as eye infections, severe rash, painful skin lesions, and neurological problems.
  • Treatment primarily involves supportive care to manage symptoms. In severe cases, patients may receive the antiviral medication TPOXX (tecovirimat).