How Severe Is the Mpox Strain Confirmed in California?

A reportedly deadlier strain of mpox has been recorded in the U.S. for the first time, the U.S. Centers for Disease Control and Prevention (CDC) has announced.
This clade I mpox outbreak first arose in the Democratic Republic of Congo and has since spread to several areas around the globe this year, including Central and Eastern Africa, Thailand, Sweden and the U.K.
While an outbreak of clade II mpox has been seen in the U.S. and elsewhere since 2022, the first U.S. clade I case was confirmed this weekend by the California Department of Public Health (CDPH).
“CDPH has reported to the Centers for Disease Control and Prevention (CDC) the first case of clade I mpox in the United States. Clade I is different than clade II, the strain of mpox that has been circulating in California and the United States since 2022,” the CDPH said. “This case was confirmed in an individual who recently traveled from Africa and is related to the ongoing outbreak of clade I mpox in Central and Eastern Africa.”
Mpox is a viral infection that causes painful skin lesions. The primary mode of transmission is via direct contact with these skin lesions or body fluids, which can include sexual contact or other forms of close physical contact with humans or animals. Men who have sex with men are at particular risk of catching the virus, however.
The CDC says that “data from affected countries indicate that a large proportion of clade I mpox cases among adults were associated with heterosexual contact.”
So how dangerous is clade I, compared with clade II?
Clade I and clade II are the two major genetic groups of the Mpox virus, a zoonotic virus in the same family as smallpox (orthopoxvirus). Clade I originates mainly in Central Africa and tends to cause more severe disease, compared with clade II, which is associated with West Africa. The CDC says clade I has historically caused more deaths than clade II, with up to 10 percent of patients dying, but cases outside of Africa appear to have a much lower fatality rate.
The CDC continues: “While outbreaks of clade I mpox used to have death rates around 3 percent-11 percent, more recent outbreaks have had death rates as low as approximately 1 percent 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 less than 1 percent both in and outside of Africa,” the CDC adds.
Of the travel-associated clade I mpox cases outside of Africa—which are all caused by the clade I subclade Ib—no deaths have been recorded.
“Clade Ib mpox arose in the Democratic Republic of Congo (DRC) just over a year ago. Both clades Ia and Ib are focussed in the DRC,” Paul Hunter, a professor of medicine at the U.K.’s University of East Anglia, said in a statement.
He went on: “Clade Ia predominantly affects children and it is believed that many of these infections are acquired from contact with infected animals. Clade Ib is mostly spreading between adults through intimate and sexual contact. Recently, however, clade Ib does seem to be affecting children more, at least within the DRC.”
The CDC says that the “risk of clade I mpox to the public in the United States remains low.”
This year, over 12,000 cases of clade I mpox have been recorded between January 1 and November 15, with at least 47 deaths occurring across Burundi, Central African Republic, the Democratic Republic of Congo, the Republic of Congo, Rwanda and Uganda.
“Historically, only Cameroon in West Africa had cases of both clade I and clade II mpox, but they were seen in different parts of the country,” the CDC says. “To date, no other country has reported cases due to both clades, with the exception of the recent travel-associated clade I cases in Germany, India, Sweden, Thailand, the United Kingdom, Zambia, and Zimbabwe.”
Fortunately, clade I mpox seems to be preventable, thanks to the Jynneos vaccine that was widely distributed over the past few years in the wake of the clade II outbreak.
“Mpox is not that infectious unless with very close contact,” Hunter said. “Early reports were that clade 1 infections are more severe than clade 2 infections and this caused particular concern. However, it is not clear how much of that difference in severity was driven by the poorer health care facilities in DRC compared to West Africa or by the intrinsic virulence of the virus.”
He went on: “It is likely that if mpox clade 1b does spread in Europe, it would spread predominantly in the same sexual networks that clade 2b spreads in. It is unlikely that we will see a similar epidemic with clade 1b as we saw with clade 2b. The reason for this is that vaccine given for 2b is also effective against 1b and people who have had an infection with 2b will also have reasonable protection against 1b, though not 100 percent.”
The clade I patient in California is currently recovering at home.
The CDPH said: “The affected individual received health care in San Mateo County based on their travel history and symptoms. The individual is isolating at home and recovering. People who had close contact with this individual are being contacted by public health workers, but there is no concern or evidence that mpox clade I is currently spreading between individuals in California or the United States.”
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