The study raises further questions about the effectiveness of the monkeypox vaccine

A new study published in JAMA documents 90 monkeypox cases after one or two doses of Jynneos vaccine – including 2 infections 3 weeks or more after two doses – among more than 7,000 vaccine recipients.

Most post-vaccination cases occurred shortly after recipients received only the initial dose of Jynneos vaccine, showing that the full efficacy of the vaccine was not achieved until 14 days or more after the second dose.

8 cases were detected 1 month after vaccination

In the study, which concluded this summer in the Chicago area, researchers tracked 90 people who tested positive for monkeypox after receiving Jynneos among 7,339 people who received a first dose of the vaccine.

Of these, 69 (77%) tested positive for monkeypox within 14 days of their first dose. Another 21 became infected after 14 days and 8 after 28 days. Only 2 people developed monkeypox 3 weeks or more after receiving both doses.

The study authors did not specify how many people in the study received two doses of the vaccine, but six of the eight people who contracted monkeypox 29 days or more after the first dose had received a second dose.

Of the 90 vaccine recipients who tested positive for monkeypox, the median age was 33, 91% were cisgender males, and 36.8% were living with HIV.

Of the cases recorded 1 to 14 days after vaccination, 36.2% (25/69) were in persons with HIV, of whom 96% (24/25) were virologically suppressed. Fifty-four percent (24/44) of early post-vaccination cases without HIV infection used pre-exposure prophylaxis.

“Because the incubation period for monkeypox is 3 to 17 days, some of the cases occurring between 1 and 14 days after vaccination may not represent true vaccination failure, since patients may have requested vaccination after realizing that they were exposed,” the authors concluded.

They added: “Of concern, at least 2 breakthrough infections have been observed in individuals at least 3 weeks after a second dose.”

The study does not address other risk factors

Michael T. Osterholm, PhD, MPH, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, editor of CIDRAP News, said the study continues to challenge the idea that one dose of Jynneos provides good protection against infection.

But like other recent studies trying to gauge how effective Jynneos is during the current outbreak, this study doesn’t consider other factors that may have protected people from contracting monkeypox.

“We can’t use these types of studies to clarify how well the vaccine works,” Osterholm said. “We have to consider risk factors like having the same number of sex partners.”

Osterholm said a key question is assessing whether people searching for jynneos are also limiting their exposure. In this case, the researchers cannot determine what the infection rates would be regardless of vaccination status.

“We also have to deal with HIV,” Osterholm said. “How well does the vaccine work in HIV-infected people undergoing prophylaxis? [preventive medication]and those who aren’t?”

US cases near 26,000

Over the weekend, the U.S. Centers for Disease Control and Prevention (CDC) reported 238 more cases of monkeypox, bringing the nationwide total to 25,851 cases.

In the global outbreak news, Vietnam has discovered its first case of monkeypox involving a woman who had traveled to Dubai.

Nigeria reported 41 new cases of monkeypox last week. The country has registered 815 cases this year, plus 7 deaths.

Finally, Bavarian Nordic, maker of Jynneos, announced today that it has signed a deal with an unnamed country in Latin America for the supply of monkeypox vaccine. This is the first agreement the company has reached with a Latin American country outside of agreements with the Pan American Health Organization (PAHO).