California Confirms First Known Case of Clade I Mpox

On Nov. 16, the California Department of Public Health confirmed, through laboratory testing, the first known case of clade I mpox in the United States. This case is related to an ongoing outbreak of clade I mpox in Central and Eastern Africa. The risk of clade I mpox to the public remains low, and there continue to be sporadic clade II mpox cases in the United States.

An announcement on the case said, “The case was diagnosed in a person who recently traveled from Eastern Africa. The individual was treated shortly after returning to the United States at a local medical facility and released. Since then, the person has isolated at home, is not on treatment specific for mpox, and symptoms are improving. Based on their travel history and symptoms, patient specimens were tested and confirmed for the presence of clade I monkeypox virus. Specimens are being sent to CDC for additional viral characterization. Additionally, CDC is working with the state to identify and follow up with potential contacts”

Casual contact, like you might have during travel, is unlikely to pose significant risks for transmission of mpox. While investigations continue into this case, CDC guidance has not changed. CDC recommends protecting yourself from mpox by:

• Avoiding close contact with people who are sick with symptoms of mpox, including those with skin or genital lesions
• Avoiding contact with contaminated materials used by people who are sick (such as clothing, bedding, toothbrushes, sex toys, or materials used in healthcare settings)
• If you're eligible, get both recommended doses of mpox vaccine

“There are two types of mpox, clade I (with subclades Ia and Ib) and clade II (with subclades IIa and IIb; IIb caused the ongoing global outbreak),” the announcement added. “You can't tell which type of mpox someone has by looking at them. Outbreaks from the different subclades can have different characteristics, like who they affect, how they're spread, or how many deaths they cause. Although clade II mpox has been circulating in the United States since 2022, clade I mpox has never been reported in the United States before now. Travel-associated cases of subclade Ib have been reported in Germany, India, Kenya, Sweden, Thailand, Zimbabwe, and the United Kingdom. Historically, clade I mpox has caused more severe illness and deaths than clade II mpox; however, recent data demonstrate that infections from clade I mpox in the current outbreak may not be as clinically severe as in previous outbreaks. While outbreaks of clade I mpox used to have death rates around 3%-11%, more recent outbreaks have had death rates as low as approximately 1% when patients received good medical oversight and supportive clinical care. Death rates are expected to be much lower in countries with stronger healthcare systems and treatment options, including the United States. Current data supports that subclade Ib has a lower death rate of < 1% both in and outside of Africa. The recent travel-associated clade I mpox cases outside of Africa have all been attributed to subclade Ib; there have been no deaths associated with these cases and available data for a subset has detailed relatively mild disease courses.”

People with mpox may get a rash that could be located on hands, feet, chest, face, mouth and/or near the genitals, including penis, testicles, labia, vagina, and anus. The incubation period is 3–17 days. During this time, a person does not have symptoms and may feel fine.

The anticipated overall risk of clade I mpox to the general population in the United States from the outbreak in Central and Eastern Africa is low. Earlier this year CDC conducted a risk assessment which included epidemiologic data from Central and Eastern Africa, data from the ongoing mpox outbreak in the United States caused by clade IIb, and historical data on clade I mpox outbreaks in DRC and other affected countries. In addition, CDC has simulated clade I mpox outbreaks. These simulations indicate that close-contact transmissions within and between households are unlikely to result in a large number of mpox clade I cases in the United States.

Additionally, in Sweden, Thailand, Germany, and India there was no apparent onward spread of the virus and the onward spread in the United Kingdom has been limited to close, household contacts so far.