Jynneos mpox vaccine provided strong protection against infection, new CDC data show

People who received one or two doses of mpox vaccine contracted the infection at substantially lower rates than unvaccinated people, a study published Thursday by the Centers for Disease Control and Prevention suggested.

The analysis, published in CDC’s online journal Morbidity and Mortality Weekly Report, showed that unvaccinated people were 9.6 times more likely than fully vaccinated people to develop mpox. The incidence of infection was 7.4 times higher in unvaccinated people than in people who had received a single dose of the vaccine. The vaccinated participants had received their most recent shot at least 14 days prior to having been infected.

The estimates were based on data on 9,544 reported mpox cases among men aged 18 to 49 who were diagnosed between July 31 and Oct. 1.

The CDC study used data from 43 jurisdictions across the country. It found no difference in effectiveness between subcutaneous (under the skin) and intradermal (into the skin) administration of the vaccine. Intradermal vaccination requires substantially less vaccine per person, allowing one standard dose to be split into five intradermal doses.

In separate data posted to the CDC’s website, the agency reported that the effectiveness of two doses of the Jynneos vaccine was 87% in people who received one dose by subcutaneous administration and one dose by intradermal administration. The data suggested the effectiveness of a single dose of the vaccine was 37%, substantially lower than a recent study conducted by scientists at the United Kingdom Health Security Agency, which estimated that the effectiveness of a single dose was 78%.

The Biden administration moved in early August to recommend intradermal administration of the vaccine as a means of stretching out limited supplies as public health authorities rushed to try to contain spread of mpox, which was transmitting primarily among gay, bisexual, and other men who have sex with men.

The fact that intradermal administration wasn’t less effective than subcutaneous dosing is good news, though it remains to be seen whether there are differences in terms of the durability of the protection generated by the two approaches, said Jay Varma, an infectious diseases epidemiologist who is director of Weill Cornell’s Center for Pandemic Prevention and Response.

The vaccine, made by Bavarian Nordic, was first designed to protect against smallpox, a related but more dangerous virus that was declared eradicated in 1980. The vaccine was developed as a hedge against an accidental or deliberate release of smallpox.

Because of similarities in the genetic structures of the two viruses, it was believed Jynneos would also protect against mpox, which was formerly known as monkeypox; animal data supported that belief. But the large and ongoing international outbreak of mpox that was first detected in mid-May has provided the first real-world data on how well the vaccine works in people.

“We were delivering monkeypox vaccines back in June and July without really any human data of efficacy. We had immunogenicity data, but very little efficiency data,” said Anu Hazra, an infectious diseases physician at Chicago’s Howard Brown Health and an assistant professor of medicine at the University of Chicago.

“This gives us a clearer picture of how we think about Jynneos vaccine efficacy against the monkeypox virus.”

John Beigel, associate director for clinical research in the National Institute of Allergy and Infectious Diseases’ division of microbiology and infectious diseases, said the results should give people confidence in the vaccine as a tool to prevent mpox.

Hazra, Varma, and Beigel were not involved in the CDC study.

Earlier data from the CDC estimated men who were unvaccinated were 14 times more likely to develop mpox than men who had received a single dose, when the analysis was conducted two weeks after receipt of that shot.

Hazra said the difference between the current and earlier estimate could be due to the fact that infections have declined in this population. As vaccination and behavioral change have reduced the risk of contracting mpox, the benefit conveyed by vaccination can appear to have shrunk.

“I don’t want people to think that the vaccine is less efficacious than it was when they looked at it two months ago. But it’s a different [risk] landscape than it was at the end of September,” Hazra told STAT.

Beigel agreed, noting that there are challenges in studying a vaccine in a situation like this, when a randomized clinical trial cannot be conducted. In a controlled trial, people who are similar are randomly assigned to get the intervention being tested or to get a placebo. In this outbreak, there are likely important differences between the men who opted to get vaccinated and men who chose not to, he said.

“It’s not perfect, but it’s the best you can do in these circumstances,” he said. “We shouldn’t be too focused on precise numbers because of all these limitations.”

Beigel and colleagues at the NIAID are conducting a study to try to determine whether subcutaneous or intradermal administration is more effective. But results of the trial, which is now fully enrolled, aren’t expected until next summer.

The CDC also published data on the safety and the tolerability of the vaccine on Thursday. Severe reactions were rare and the rate of reported reactions were similar regardless of whether the vaccine was administered subcutaneous or intradermally. The most commonly reported adverse events were skin reactions at the injection site.

This story has been updated with additional CDC data on the vaccine’s effectiveness.

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