Small UK study shows 78% protection from 1 dose of monkeypox vaccine

New UK Health Security Agency (HSA) data that are not yet peer-reviewed indicate that a single Jynneos vaccine dose provides 78% protection against monkeypox 14 days or more after vaccination, but the number of vaccinated men in the study was small.

For the study, published yesterday, HSA scientists and their UK colleagues analyzed monkeypox cases in England from Jul 4 to Nov 3 involving men who have sex with men at high risk for the disease. Of the 363 monkeypox patients during this period, 8 had been vaccinated at least 14 days before, and 32 had been vaccinated between 0 to 13 days before. The other 323 had not received a Jynneos vaccine.

The authors said the estimated vaccine effectiveness (VE) 14 or more days after a single dose was 78% (95% confidence interval [CI], 54% to 89%). For men vaccinated 13 days or less, VE was -4% (95% CI, -50% to 29%).

Senior author Jamie Lopez-Bernal, MBBS, MScPH, consultant HSA epidemiologist, said in an agency news release, "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. A second dose is expected to offer even greater and longer lasting protection."

The authors note that the results are similar to findings from an Israeli study in late September that was also not peer-reviewed. Also, US data published around the same time showed that at-risk unvaccinated people have a 14-times-greater risk of contracting the disease. VE data published so far, however, have not factored in people's changing behavior.

At the time of the US study, Michael T. Osterholm, PhD, MPH, director of the University of Minnesota's Center for Infectious Disease Research and Policy, publisher of CIDRAP News, said there were too many unaccounted-for variables in the data. "People who seek out vaccines may be behaving differently from people who don't," he explained. "Did they also take a pause on sexual behavior?"

In fact, the authors of the new study write, "Behavioural changes post-vaccination may also have affected VE estimates; for example, if vaccinated individuals were less likely to abstain from high-risk sexual activity as a result of having received the vaccine then the pharmacological effectiveness would be underestimated."

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