A new study published in JAMA documents 90 monkeypox cases after one or two doses of the 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 the recipients had received only the initial dose of Jynneos vaccine, demonstrating full vaccine efficacy was not reached until 14 or more days after the second dose.
8 cases noted 1 month post-vaccine
In the study, which was completed in the Chicago area this summer, researchers tracked 90 people who tested positive for monkeypox after receiving Jynneos among 7,339 who got a first dose of the vaccine.
Of them, 69 (77%) tested positive for monkeypox within 14 days after their first dose. Another 21 got infected after 14 days, and 8 after 28 days. Only 2 people contracted monkeypox 3 weeks or more after receiving both doses.
The study authors don't specify how many people in the study received two vaccine doses, but six of the eight people who contracted monkeypox 29 or more days 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 men, and 36.8% were living with HIV.
Of the cases recorded 1 to 14 days after vaccination, 36.2% (25/69) involved people with HIV, of whom 96% (24/25) were virologically suppressed. Fifty-four percent (24/44) of early post-vaccination cases without HIV infection were using 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 vaccine failure because patients may have sought vaccination after realizing they were exposed," the authors concluded.
They added, "Of concern is that at least 2 breakthrough infections were observed in individuals at least 3 weeks after a second dose."
Study doesn't address other risk factors
Michael T. Osterholm, PhD, MPH, director of the University of Minnesota's Center for Infectious Disease Research and Policy, publisher 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 that attempt to gauge how effective Jynneos is during the current outbreak, this study does not account for other factors that may have protected people from contracting monkeypox.
"We can't clarify how well the vaccine is working based on these kinds of studies," Osterholm said. "We have to adjust for risk factors, like equivalent number of sex partners."
Osterholm said a key question to address is to assess if people seeking out Jynneos are also limiting their exposure. If so, researchers cannot determine what the infection rates would be regardless of vaccination status.
"We also have to look at HIV," Osterholm said. "How well does the vaccine work in those with HIV, those who are on prophylaxis [preventive medication], and those who are not?"
US cases approach 26,000
Over the weekend the US Centers for Disease Control and Prevention (CDC) reported 238 more monkeypox cases, raising the national total to 25,851 cases.
In global outbreak news, Vietnam has detected its first monkeypox case, which involves a woman who had traveled to Dubai.
Nigeria reported 41 new monkeypox cases over the past week. The country has recorded 815 cases this year, plus 7 deaths.
Finally, Bavarian Nordic, maker of Jynneos, said today it has signed a contract with an unnamed country in Latin America to supply monkeypox vaccine. This is the first agreement the company has made with a Latin American country outside of agreements with the Pan American Health Organization (PAHO).