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The Centers for Disease Control and Prevention expressed concern Tuesday about an unusual outbreak of monkeypox in the United Kingdom, suggesting there appears to be at least some undetected transmission of the virus there and warning of the possibility that the outbreak could spread beyond U.K. borders.

“We do have a level of concern that this is very different than what we typically think of from monkeypox. And I think we have some concern that there could be spread outside the U.K associated with this,” Jennifer McQuiston, a senior CDC official, told STAT in an interview.


Seven confirmed and one probable case of monkeypox have been discovered in the U.K. since early May — an unusually large number given that human monkeypox cases are uncommon, and are especially rare outside West and Central Africa. While one of the cases had recently traveled to Nigeria, where monkeypox is endemic, the others appear to have contracted the virus in the U.K. None of the people infected domestically has any known connections with the traveler and the timing of the onset of the cases suggest he was not the source of those infections.

Complicating the situation — and amplifying the concern — is the fact that the other cases comprise two distinct groups that have no discernible links to one other. Two of the confirmed cases and the single probable case are a family unit, Maria Van Kerkhove, a World Health Organization expert, said at a press conference on Tuesday. The other four confirmed cases, disclosed most recently, were identified by the U.K. Health Security Agency as gay, bisexual, or other men who have sex with men — three from London and a fourth from Newcastle, nearly 300 miles to the north. All four were likely infected in London, the agency said.

It is not known how any of these people contracted the virus. Transmission is thought to occur mainly through virus-laced droplets, but direct contact with lesions or bodily fluids from an infected person, or indirect contact via contaminated clothing or linens, can also result in transmission.


The fact that there are two apparently unconnected clusters suggests there may be more than one chain of transmission in the country, each of which could contain additional cases that haven’t yet been detected. Van Kerkhove, who leads the emerging diseases and zoonoses unit in the WHO’s Health Emergencies Program, said virus sequencing is underway in the U.K. to try to determine if the cases are genetically linked.

There is no suggestion that monkeypox is spreading to large numbers of people in the U.K. Still, the evidence that there are likely some undetected cases is fueling worry about how much transmission there may be there, and whether the virus has traveled further afield.

“You have two clusters that have no link to travel or to other people who are known to be associated with a recognized outbreak. It suggests that there are unknown chains of transmission happening,” McQuiston said. “If there appears to be unknown chains of transmission, it just puts us on alert to be thinking: Could this be spreading outside the U.K.?”

Although human monkeypox cases outside Africa are rare, in recent years there has been a spate of exported cases to the United States (two in 2021), the U.K., Israel, and Singapore. The four most recent cases were infected with the West African clade of monkeypox, which causes milder disease than the Central African clade, the UKHSA said.

Monkeypox has symptoms similar to but milder than smallpox, which was declared eradicated in 1980. In addition to flu-like symptoms, infection triggers a distinctive rash. Many conditions can cause rashes but the monkeypox rash has some unusual features, notably the fact that vesicles can form on the palms of the hands.

There is a vaccine that was licensed in the U.S. in 2019 for people 18 years of age and older to protect against smallpox and monkeypox, Bavarian Nordic’s Jynneos. A second vaccine, ACAM2000 made by Emergent Product Development, protects against smallpox and is thought to offer some protection against monkeypox. But both vaccines are licensed only for people considered at high risk of contracting the disease. In a 2003 outbreak in the U.S. that involved 47 people, smallpox vaccine was used.

In Africa, monkeypox has been fatal in about 1 in 10 cases, with severe disease and death more likely among children.

Because of the discovery of cases among people who are gay, bisexual, or other men who have sex with men, public health authorities in the U.K. have alerted sexual health clinics to be on the lookout for people with unexplained rashes. McQuiston said the CDC is considering sending out a health alert to medical professions and sexually transmitted infection clinics in this country.

“There’s a lot of travel between the U.K. and the United States and other global areas. So I think our concern is that given that you do have four cases among men who have sex with men, that we probably need to be thinking about messaging to our STI clinics … about what to be on the lookout for, what to be alert for,” she said.

Van Kerkhove said the WHO has reached out to European countries through its Copenhagen-based European office to raise awareness of the risk of finding monkeypox cases.

Communications around the outbreak have to be crafted with care, McQuiston said.

“This is certainly a population that has been stigmatized for sexually transmitted infections like HIV in the past. And I think we have learned a bit from those experiences about how to approach this with the type of sensitivity it requires,” she said.

Public health investigators in the U.K. are working to trace who the confirmed cases were in contact with before they became ill, to try to find the source of their infections. But they’re also seeking to identify contacts of the infected to see if they have transmitted the virus to others.

McQuiston said to date the CDC has not been notified that any Americans have turned up on the UKHSA’s contact list for these cases. But she did reveal that six people in the United States are being monitored because they were on the same flight as the man who traveled from Nigeria to the U.K. on May 4. Though there’s no suggestion the individuals had direct contact with the man — they sat within a three-row radius of his seat — the six individuals will be monitored for 21 days, she said.

The U.K. outbreak is a reminder of the need to pay more attention to monkeypox, the disease dynamics of which are poorly understood, said Ibrahima Socé Fall, the WHO’s assistant director-general for emergency response.

“Clearly the main problem we need to investigate is really knowing the real extent of monkeypox transmission in Africa and making sure that we invest in tools for prevention and treatment for people who are the most exposed in Africa,” he said.

Correction: An earlier version of this article mistakenly said there was no vaccine licensed to protect against monkeypox.  

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