An analysis released Tuesday by U.K. health officials indicates that even one dose of the monkeypox vaccine provides strong protection against the virus.
Researchers at the U.K. Health Security Agency estimated that one dose of the vaccine was 78% effective at protecting against infection 14 or more days after vaccination.
The findings bolster the case that the vaccination drive has helped combat this year’s global outbreak of monkeypox, which has predominantly affected men who have sex with men.
The analysis was done by comparing infection rates in people who were vaccinated versus those who were eligible but who did not receive a vaccine. The researchers found that out of the 363 infections reported from July 4 to Nov. 3, eight occurred in people who’d been vaccinated at least two weeks earlier. Thirty-two cases had been vaccinated less than two weeks earlier, and the rest — 323 cases — had not been vaccinated.
U.K. officials said they are continuing to investigate the duration of that protection and the level of protection provided by two doses. The monkeypox vaccine is made by the Danish company Bavarian Nordic and is sold in the United States as Jynneos. It’s a two-dose vaccine with the doses given 28 days apart.
“We now know that a single vaccine dose provides strong protection against monkeypox, which shows just how important vaccination is to protect yourself and others,” Jamie Lopez-Bernal, a consultant epidemiologist at UKHSA, said in a statement. “A second dose is expected to offer even greater and longer lasting protection.”
The findings from the U.K. echo a report from September from the U.S. Centers for Disease Control and Prevention, which also found that one dose of the monkeypox vaccine reduced the risk of infection. The CDC reported that people who were eligible for the vaccine but had not gotten the shot were 14 times more likely to contract the virus than people who were vaccinated.
Experts have cautioned that these data are preliminary and that there are limitations. These types of analyses, for example, don’t take into account behavior. Perhaps people who get vaccinated also took other precautions to avoid infection during the height of the outbreak, like reducing their number of sex partners. That could make it look like the vaccine is more protective than it is.
An earlier study by researchers in the Netherlands had raised doubts about the level of protection provided by the vaccine, even with two doses. The research found the vaccine elicited low levels of neutralizing antibodies that target the virus, and that the antibodies weren’t robust neutralizers.
The global monkeypox outbreak that started in the spring was unprecedented. Typically, the virus has not been seen spreading outside a handful of countries in West and Central Africa. But this year, the virus took off, with nearly 80,000 infections reported to the World Health Organization as of Nov. 13, including 50 fatal cases. The vast majority of the cases were documented in Europe and the Americas. The United States has reported more than 29,000 cases.
The vast majority of the infections were reported in men who have sex with men, particularly those who had multiple sex partners.
But since the late summer and early fall, new cases have declined dramatically. In the United States, for example, the average number of daily cases in early August was 450. Most recently, there was an average of 13 new cases a day.
The collapse in cases has been attributed to the vaccination campaign, changes in behavior among those most at-risk (essentially, practicing safer sex and reducing the number of sex partners), and immunity built up in the population that was most at risk.