CDC Says U.S. Mpox May Be Moving Toward Endemic Circulation

A new CDC analysis says mpox continued circulating at low levels across the United States in 2024 and 2025, a pattern that may signal a shift toward endemic circulation rather than a return to the sharp nationwide outbreak seen in 2022.

The report, published August 20, 2026, in the CDC’s Morbidity and Mortality Weekly Report, identified 5,322 reported U.S. cases during the two-year study period: 2,803 in 2024 and 2,519 in 2025.

What changed in 2024 and 2025

CDC researchers found that clade IIb, the subclade associated with the 2022 U.S. outbreak and continued domestic transmission, remained present at low levels rather than disappearing.

Reported cases rose during part of 2025. The CDC counted 873 cases during September and October 2025, compared with 423 during the same two months in 2024. Cases then declined to 448 during November and December 2025, returning to background levels by the end of December, according to the report.

The geographic pattern was also broad. Cases affected 364 counties or municipalities in 2024 and 326 in 2025. The CDC did not describe the pattern as a new nationwide surge. Instead, researchers said the combination of persistent transmission and spread across hundreds of jurisdictions suggests a possible transition toward endemic circulation.

What “endemic circulation” means

In this context, endemic circulation means a disease remains present through ongoing, lower-level transmission rather than appearing only in a short-lived outbreak. The CDC’s wording is cautious: It does not say mpox has definitively become endemic in the United States.

Current levels also remain far below the 2022 peak. The CDC reported that clade II cases reached approximately 3,000 per week during that outbreak, while the current analysis describes persistent background transmission—not a repeat of the rapid rise that prompted widespread public-health action.

Vaccination and hospitalization

The report also examined hospitalization risk. Among 860 cases with available information, unvaccinated patients had 9.73 times the odds of hospitalization compared with patients who had completed the two-dose JYNNEOS vaccination series, after the analysis controlled for age group and HIV status. The 95% confidence interval ranged from 2.93 to 60.65.

That estimate should be interpreted carefully. It covered only 39% of the U.S. cases in the analysis, and vaccination status was missing for most patients. The finding shows an association in the CDC analysis; it does not, by itself, prove that vaccination caused the lower hospitalization odds.

People who may be at elevated risk should check whether they are eligible for the two-dose JYNNEOS series. Anyone with symptoms or a possible exposure should contact a health care provider or local health department for testing, treatment and prevention guidance.

Clade IIb remains the main domestic concern

The report distinguishes persistent clade IIb circulation from clade Ib cases identified in the United States. The CDC reported 11 clade Ib cases during 2024 and 2025. Most were associated with travel or links to travelers, and the report found no sustained clade Ib transmission during that period.

Two clade Ib sequences were part of a geographically limited domestic cluster in late 2025. The CDC said that local transmission involved three cases linked to a travel-associated introduction and did not result in further spread.

CDC guidance says clade II is circulating at low levels. It also says the risk from clade I to most people in the United States remains low.

What officials are watching next

The CDC is emphasizing continued case reporting, genetic sequencing, focused education and vaccination coverage. Sequencing can help public-health officials detect changes in transmission and identify imported or emerging clades.

The analysis also has limits. Passive surveillance may miss mild infections for which people do not seek care, so reported cases may underestimate the number of infections. Recent CDC case counts are also subject to reporting delays and should be treated as preliminary when the agency labels them that way.

For readers, the main takeaway is that mpox did not disappear after the 2022 outbreak. The latest CDC evidence points to low-level, geographically dispersed clade IIb transmission and a possible shift toward persistent background circulation—an important surveillance concern, but not an announcement of a new emergency.

Sources

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