Jynneos vaccine offers protection against Mpox, new studies confirm

Why it matters: A resurgence is possible.

In the United States, there have been more than 30,000 cases of mpox in the past year, with cases plummeting after last summer’s peak. Mpox cases have also declined globally, and the World Health Organization ended the public health emergency earlier this month.

“But the outbreak is not over and we must remain alert and continue our prevention efforts,” said Dr. Christopher Braden, the mpox response incident manager at the CDC, at a news conference Thursday.

More than 1.2 million doses of the vaccine have been administered in the United States in the past year. But the number of doses administered has since declined last summer, and nationwide only 23 percent of people those deemed at risk have been fully vaccinated, according to the CDC. There are also major geographic, racial and ethnic differences in vaccination coverage.

And it is clear that the vaccine is not a panacea. In a new cluster of cases recently reported in Chicago, many of the patients were fully vaccinated.

The Chicago Department of Health and the CDC are currently investigating this cluster, which now includes 21 people, all of whom have had mild symptoms, said Dr. Demetre Daskalakis, the White House deputy coordinator for national MPOX responses, during the briefing on Thursday.

“What we do know, however, is that vaccination makes getting and spreading mpox less likely and, more importantly, can reduce the likelihood of serious illness, hospitalization and death, even if it doesn’t prevent infection,” he said.

Background: The vaccine was largely unknown.

Jynneos, manufactured by a small Danish company, is the safest of the two vaccines available for mpox. It was initially planned to be given in two doses, both injected under the skin, 28 days apart.

However, because supplies of the vaccine were limited, officials deviated from the intended regimen.

Some started by administering just a single dose of the vaccine, which some studies had suggested could provide significant protection. Then, last August, federal officials issued an emergency intradermal dosing authorization to increase the available supply.

But there is little evidence for the effectiveness of these strategies, which were largely based on research findings rather than patients’ real-life experiences.

What’s Next: Officials are preparing a new push for vaccination.

The number of cases may increase in the coming weeks “as people gather for festivals and other events,” says the CDC has warned. Public health experts and officials are urging people at risk to get vaccinated before Pride events begin next month.

“For me, the most important message would be that if you haven’t received a dose yet, get it now, because you have to wait four weeks before you get your second dose,” said Dr. Jacqueline L. Gerhart, the chief physician at Epic Research and one of the principal investigators on the NEJM study.

Additional studies of the vaccine, including research into how well it holds up over time, are currently underway, said Dr. Daskalakis.

Sharon Otterman reporting contributed.