For the first time since the first American cases were reported last year, the United States has tallied no new cases of mpox for more than a week, according to figures published Wednesday by the Centers for Disease Control and Prevention.
The milestone comes after cases of mpox, formerly called monkeypox, had begun to slow last year off a summer peak of nearly 500 infections daily around the country.
It is unclear how many weeks of no new cases are needed until the outbreak can be declared officially over in the U.S. The Biden administration moved late last year to end the public health emergency it had declared for mpox, when cases were averaging 16 per day.
A CDC spokesperson did not immediately return a request for comment.
Mpox resulted in painful, weeks-long lesions for most people who were infected by the virus. More than 30,000 cases and 42 deaths from mpox have been recorded by the CDC since the outbreak began in May 2022.
Mpox cases have largely been among men who have sex with men. Severe cases have disproportionately affected communities of color: 87% of deaths were in Black men, the CDC reported through March, many who were immunocompromised with advanced HIV.
Why mpox cases slowed
Scientists have said that so-called “network immunity” may have helped drive an end to the mpox outbreak, with people either dying or developing immunity from infections across the sexual networks that first spread the virus.
“Infection-induced immunity of the individuals at the core of the sexual network generated ‘network immunity’ which halted the epidemic,” Dr. Christophe Van Dijck of the Institute of Tropical Medicine in Belgium theorized earlier this month, in a release discussing research presented at the European Congress of Clinical Microbiology and Infectious Diseases.
Authorities have previously credited the slowdown to other factors as well, like changes in behavior or the rollout of Jynneos vaccinations, which are estimated to have curbed infections.
However, officials also acknowledged these fell short of explaining the entire story. Behavior changes were not uniform and cases plummeted even in countries without any vaccines.
A CDC analysis presented at the agency’s Epidemic Intelligence Service conference this week found mpox transmission “was modestly lower” after cases peaked across several middle and high-income countries that rolled out vaccines, compared with those that did not.
That difference was statistically significant, though they cautioned more data was needed to confirm “causality about the effect of vaccination campaigns” on the outbreak.
“An alternate explanation could be that countries with vaccination also mounted more aggressive responses overall, with strong campaigns to change behavior and raise awareness,” the CDC’s Andrea Stewart said at the conference.
Stewart said others at the CDC are working on “more advanced modeling” that might also control for whether the pace at which cases swelled earlier in the outbreak may have affected how infections later slowed.
“Countries that experienced more aggressive outbreaks early may have been more likely to implement vaccine campaigns early as well. Their outbreaks could have declined quickly due to population dynamics,” said Stewart.
Risk of resurgence?
U.S. health officials have warned that the country could still see a resurgence over the coming months, with renewed outbreaks as travel and large gatherings pick up in the warmer months.
“If mpox reintroduction occurs and no additional vaccination or sexual behavior adaptations occur, the risk of a resurgent mpox outbreak is greater than 35% in most jurisdictions in the United States,” the CDC’s modelers concluded in April.
New cases continue to be reported abroad. And at least one site in the United States – located in central Virginia – has continued to turn up “consistent detection” of mpox in wastewater samples collected by the CDC from sewers there as recently as mid-April, despite no new reported cases for months.
Virginia’s own wastewater surveillance has not detected mpox in any other parts of the state across the 36 monitoring sites set up in March, Cheryle Rodriguez, a spokesperson for the state’s health department, said in an email.
The consistent detection in central Virginia could be the result of previously diagnosed mpox infections that have yet to resolve, she said, or new infections that have gone unreported to state authorities.
“The most recently reported case of mpox in the Central Region of Virginia occurred in November 2022,” said Rodriguez.
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