California has confirmed the first case of a new mpox strain that is spreading from person to person. The infected individual had recently traveled from Eastern Africa, where multiple countries are currently dealing with the transmission of this virus.
The infected person received treatment at a health facility in San Mateo County and is now in isolation at home, according to a statement released by the California Department of Public Health on Saturday. The Centers for Disease Control and Prevention also noted that the individual is not currently undergoing treatment with anti-mpox medication, suggesting that the infection may be mild.
The virus strain detected in the individual is known as clade Ib, a variation of clade I viruses that typically originate from an animal source in East African countries where the virus is naturally present. This strain is distinct from the clade IIb virus responsible for the major international mpox outbreak that began in 2022. There is currently no evidence that the infected person has transmitted the virus to others.
The World Health Organization declared the ongoing person-to-person spread of mpox a Public Health Emergency of International Concern in August. Specimens from the California case have been sent to the CDC for further analysis.
The spread of clade Ib viruses from person to person was first identified in September 2023 in the Democratic Republic of the Congo. While most transmissions are occurring in African nations, cases related to travel have been reported in several countries, including Germany, India, Kenya, Sweden, Thailand, Zimbabwe, the United Kingdom, and now the United States.
The CDC has assessed the risk posed by the clade Ib outbreak in Central and Eastern Africa to the U.S. population as low, based on simulation exercises conducted. The simulations suggest that close-contact transmissions within households and between individuals are unlikely to lead to a large number of mpox clade I cases in the United States.
As of this year, over 115,000 mpox cases, including both clade I and clade II strains, have been reported in at least 123 countries. Of these cases, at least 255 have been fatal. Mpox virus shares similarities with smallpox, a now-eradicated virus. Both viruses cause a blistering rash that can result in scarring, with mpox having a lower fatality rate compared to smallpox.
Historically, mpox cases were believed to be contracted through contact with infected animals in African regions where the viruses are prevalent. However, since 2022, it has been observed that sexual transmission of the viruses is occurring, particularly within communities of gay, bisexual, and other men who have sex with men. Vaccines like Bavarian Nordic’s Jynneos vaccine offer protection against mpox, but global production of these vaccines is limited.
