More than two years after the mpox outbreak was officially declared over in the United States, experts in infectious diseases are expressing worry over a new uptick in cases in California.
The outbreak, which primarily spread among men who have sex with men, was considered resolved in early 2023, yet there has been ongoing low-level transmission since then.
Recent identification of three unrelated mpox cases in California is raising eyebrows because these involve a more contagious and dangerous variant known as clade I mpox.
During the 2022 outbreak, the strains that circulated in the U.S. were clade II, which is associated with a relatively low mortality rate—only 58 deaths out of more than 32,000 recorded infections, resulting in a rate of around 0.2 percent.
While both clades are found in Central Africa, clade I is historically linked with higher severity and mortality. It typically leads to more serious diseases.
All three recent cases in California led to hospitalization, which the California Department of Public Health (CDPH) highlights as a sign that the virus is moving through communities of men who have sex with men, echoing patterns from the 2022 outbreak.
Joseph Chellaby, a member of the HIV Medical Association’s board and an assistant professor at Washington University in St. Louis, mentioned that clade I mpox’s arrival in the U.S. is likely just a matter of time.
He pointed out that there have been past instances of clade I mpox in California, usually linked to recent travelers from Africa. However, none of the confirmed cases were tied to any recent travel.
“As epidemiologists and public health officials, this makes us a little uneasy, as it suggests there’s community spread of clade I that we might not be detecting,” Chellaby shared.
Mpox mainly spreads through close skin-to-skin contact, particularly involving the painful rash characteristic of the illness. Given that the mode of transmission isn’t yet fully understood, Chellaby believes there are “probably many more cases” yet to be identified.
Despite the new cases, health officials, including those at CDPH, reassure that the overall risk to the general public remains low.
Seth Blumberg, a medicine professor at the University of California, San Francisco, remarked that a somewhat encouraging sign is the lack of reported deaths linked to these recent infections, with a mortality rate appearing to remain low.
He added that animal-to-human transmission of clade I mpox is relatively common in Central Africa. If a zoonotic outbreak were to happen in California, it would pose an even greater public health challenge.
Based on insights from earlier outbreaks, Chellaby assessed that the likelihood of everyday contact leading to transmission is low.
“If there’s anything we learned from the 2022 outbreak, it’s that casual contact, like sharing clothing or sitting next to someone, didn’t lead to spread,” Chellaby explained. “These lesions require very close, intimate contact, which is why sexual contact was the primary mode of transmission.”
During the global emergence of mpox outbreaks in 2022, public health entities faced the challenge of limited vaccines and treatments tailored for mpox.
Nevertheless, treatments for smallpox, which also cover mpox, were put into use rapidly. These include the Jynneos vaccine and the antiviral drug Tecobirimat (Tpoxx).
Both treatments are expected to be effective against clade I mpox, similar to their use against clade II. The full protection from Jynneos necessitates two doses, while Tpoxx can be given either intravenously or orally.
A further concern for specialists in infectious diseases is the anticipated lack of federal public health support should these cases lead to a more serious situation.
The Centers for Disease Control and Prevention (CDC) has experienced a significant reduction in staff this year. This, along with the current government shutdown, raises questions about the reliability of federal public health resources at this time.
“Monitoring and testing specific to clade, along with real-time data communication, has been greatly diminished due to public health budget cuts,” Chellaby noted, highlighting recent disruptions in staff at the CDC’s Morbidity and Mortality Weekly Report.
Despite the challenges, he emphasized that California possesses a strong public health system.
