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May 18, 2023

CDC warns of summer mpox resurgence, urges vaccination for high-risk groups

After last year's outbreak, new studies show two doses of the Jynneos vaccine are effective at limiting transmission

A cluster of at least 21 mpox cases in Chicago this month prompted a new warning from the U.S. Centers for Disease Control and Prevention about a possible surge of cases this summer.

The virus, formerly known as monkeypox, infected more than 30,000 people and caused 42 deaths in the U.S. last year. Globally, the outbreak reached more than 100 countries and infected nearly 87,000 people, about one-third of them in the U.S.

For decades, mpox had largely been contained to parts of central and western Africa, where it's endemic, and there had not been any major outbreaks in the U.S. A small outbreak occurred in the U.S. in 2003, but there was no human-to-human transmission and no deaths.

Last May, the virus emerged in Europe and spread rapidly from human transmission, predominantly among gay and bisexual men, as well as transgender and non-binary people, data showed. The main mode of transmission between people is skin-to-skin contact, although it can also be transmitted through respiratory droplets.

Mpox typically begins with a fever followed by a rash on the face, inside the mouth and on other parts of the body, including the hands, feet, chest, genitals and anus. Swollen lymph nodes also may occur. Symptoms usually last for two to four weeks. The disease is rarely fatal.

Vaccination is recommended for gay and bisexual men as well as transgender and nonbinary people who have had more than one sexual partner or a new STD diagnosis in the past six months. The CDC's warning comes ahead of Pride Month, when LGBTQ events and festivals could result in more mpox transmission. Cases in the U.S. peaked last August at around 450 per day. 

Although the World Health Organization ended the global mpox emergency last week, public health officials are still urging caution heading into the summer months.

"It is important to say that without renewed prevention efforts, especially vaccination, we are definitely at risk of a resurgence, in fact a substantial risk of resurgence," Demetre Daskalakis, the White House’s national mpox response deputy coordinator, said Thursday.

The cluster of mpox cases reported in Chicago this month included people who had previously been vaccinated against the virus, either partially or fully. Daskalakis said the patients all have mild symptoms.

Three new studies have provided more data on the efficacy of the Jynneos mpox vaccine made by Danish manufacture Bavarian Nordic. During last year's outbreak, shortages of the vaccine led to some U.S. locations offering only a single dose, instead of the recommended two doses.

The reports released Thursday by the CDC, the New York State Department of Health and Epic Research showed that two full doses of the vaccine ranged from 66% to 86% efficacy in preventing mpox. Across three studies, one full dose of the vaccine ranged from 36% to 75% efficacy in preventing mpox.

The research also found that the efficacy of the vaccine was roughly the same whether shots were administered under the skin or between layers of skin. Less of the vaccine is needed when administering between layers of the skin, which allowed health care providers to preserve vaccine supplies last year.

Daskalakis explained that even if the vaccine fails to prevent transmission, it lowers the risk of severe illness. Higher rates of vaccination in high-risk populations also reduce the overall likelihood of another outbreak.

To date, CDC officials said about 1.2 million people have received the mpox vaccine, but less than a quarter of the population considered at risk has been fully vaccinated.

The presence of cases among people in Chicago who were vaccinated raises questions about whether mpox immunity wanes over time after vaccination.

Christopher Braden, the CDC’s mpox incident manager, said Thursday it's still unclear whether immunity wanes or if the virus has evolved. The CDC is currently evaluating longer-term vaccine effectiveness and any potential changes to the virus.

Researchers at Penn State University recently developed an mpox rapid test that detects the virus within minutes. Unlike like PCR tests, which require health care providers to swab lesions and submit them for an analysis that can take several days, the rapid tests could be self-administered at home. The research team hopes to find partners to commercialize the rapid test after completing additional studies on its ability to detect the virus.

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