Early analysis suggests monkeypox vaccine is reducing risk of infection in U.S.

A very preliminary analysis of data from 32 states appears to suggest that the monkeypox vaccine being used in the United States is reducing the risk of infection among vaccinated people, Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said Wednesday.

Walensky said the analysis showed that people who were eligible to be vaccinated against monkeypox because of their personal level of risk but who had not received the vaccine were 14 times more likely to contract monkeypox than those who were vaccinated, when the analysis looked at data from two weeks after receipt of a single dose.

Outside experts cautioned against reading too much into the data, with one saying he felt it was “premature” to publish such a finding.


“It might be true. But we can’t tell from this,” Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, said of the estimate of a 14-fold reduction in infections among people who had received one dose of vaccine. “I’m really looking forward to more data.”

The vaccine, made by Bavarian Nordic and sold as Jynneos, is a two-dose vaccine, with the doses given 28 days apart. Data based on the response to second doses have not yet been generated, Walensky said.


“There have been limited data on how well the Jynneos vaccine performs against monkeypox in real-world conditions,” the CDC director said during an online press conference. “These new data provide us with a level of cautious optimism that the vaccine is working as intended.”

Despite the possible benefit of a single dose of vaccine, Walensky and other administration officials involved in the monkeypox response stressed that people should be sure to get the second dose when they are eligible.

One major reason to view the finding with caution stems from the fact that the CDC scientists who conducted the analysis could not tell if there were important differences between the vaccinated and unvaccinated people, differences that could account for some portion of the apparent benefit.

For instance, there was no way to determine if people who were motivated to get vaccinated were also moved to cut back on the number of sexual partners they had in order to reduce their risk of contracting the virus. If men who did not get vaccinated did not change risky behavior, but men who were vaccinated did, that could make it appear that the protective effect of the vaccine was larger than it actually was, Osterholm said.

Marion Koopmans, the senior author of a recent paper that raised concerns about the efficacy of the vaccine, both after one dose and after two, agreed that adjusting for possible differences between people who were vaccinated against monkeypox and those who were eligible but did not take the vaccine is critical to understanding how well it is working.

“Could they be the people that are more serious about health risks? In other words: How comparable are the groups in terms of risk behavior?” said Koopmans, who heads the department of viroscience at Erasmus Medical Center in Rotterdam, the Netherlands.

She and colleagues recently released a paper showing that even two doses of Jynneos elicit low levels of neutralizing antibodies, and those antibodies had poor neutralizing capacity. The study, which has not yet been through peer-review, suggested among other things that the immune response to a single dose would probably be inadequate to induce protection.

She admitted to being surprised by the CDC estimate, suggesting this much benefit within 14 days of a single dose of vaccine “to me seems very fast. We certainly do not see much response yet,” she said in an email.

“Based on what we saw, [I] would not have expected that big effect but of course it is the clinical response that matters,” she said, referring to the fact data generated from use of the vaccine in people trumps data generated through laboratory studies.

Walensky said the CDC’s scientists did not examine whether levels of protection appeared to differ between people who had received one full dose of the vaccine delivered subcutaneously (under the skin) and people who had received a fractional or one-fifth dose of the vaccine by intradermal (into the skin) injection. The United States moved to the latter approach in early August.

All those caveats combine to make the finding far from certain, Boghuma Titanji, an assistant professor of infectious diseases at Emory University, said on Twitter.

“Too many potential confounding factors, not enough follow-up time, no stratification by route of administration,” Titanji wrote. “There’s a trend towards a protective effect but will interpret stated numbers with caution.”

Robert Fenton, the Biden administration’s monkeypox response coordinator, said to date more than 800,000 doses of monkeypox vaccine have been administered in this country.

The administration officials announced two changes to the monkeypox vaccination program on Wednesday. The first involved who is eligible to be vaccinated. With increased availability of vaccination doses, the response is moving to a pre-exposure prophylaxis or PrEP approach, making vaccine available to anyone at risk, not just people at the highest risk.

“This new PrEP strategy means that more people who might be at present or future risk for monkeypox now qualify for the vaccine. That includes gay, bisexual, and other men who have sex with men, transgender or gender diverse people who have had more than one sex partner in the last six months, had sex in a place associated with higher monkeypox risk, or had a sexually transmitted infection diagnosed over that same time period,” said Demetre Daskalakis, deputy coordinator for the monkeypox response for the White House.

Daskalakis said the new strategy also makes people who have sex with people who fit those criteria eligible for vaccine, including commercial sex workers.

The second announced change involves where on one’s body vaccine doses are administered. Daskalakis said some people have been avoiding being vaccinated because the site of vaccination — the forearm — leaves a visible mark for some period afterwards. Some individuals would prefer to be vaccinated on a part of their bodies where it’s less likely someone else might see they’ve been vaccinated, he said.

The CDC is now saying people may be vaccinated on the shoulder or upper back, which are more likely to be covered by clothing, he said.

The United States has reported 25,300 cases of monkeypox, and one death. Walensky said the growth of new cases is declining, week over week — a trend that has been seen in a number of other jurisdictions combating the virus. Globally, 106 countries have reported nearly 66,500 cases this year, with 26 deaths.

A recent analysis from the United Kingdom’s Health Security Agenda cited modeling studies that suggest a decline in cases there is partially due to the country’s monkeypox vaccination efforts, but also to behavioral changes among men who have sex with men. It noted that the drop in new cases of monkeypox coincided with declines in the rates of two sexually transmitted diseases often transmitted among men who have sex with men.

Source: STAT