Publisher Health

WASHINGTON — Americans who received a single dose of the monkeypox vaccine were significantly less likely to be infected by the virus over the summer than those who did not, according to a study published on Wednesday by the Centers for Disease Control and Prevention that offered a limited glimpse of the shot’s protectiveness.

The findings, gleaned from 32 states between the end of July and early September, were some of the first federal data that suggested how well the Jynneos vaccine, the main shot being used to respond to the monkeypox outbreak, prevents infections. Unvaccinated people were 14 times as likely to be infected as those who received an initial shot, the research showed.

“These new data provide us with a level of cautious optimism that the vaccine is working as intended,” Dr. Rochelle P. Walensky, the C.D.C. director, said at a White House briefing, adding that “even one dose of the monkeypox vaccine offers at least some initial protection against infection.”

Though monkeypox cases have declined in recent weeks, the release of the data appeared to serve in part as an effort to reinvigorate the inoculation campaign and as a prompt to the hundreds of thousands of Americans at high risk who still have not been vaccinated.

Federal officials have attributed the decline in cases to vaccination, immunity from infection and changes in behavior among those at greater risk of infection. They have said in recent days that they hope to slow cases to a trickle, allowing them to isolate and vaccinate close contacts of infected people and stanch the outbreak in the United States.

About 804,000 doses of the vaccine had been administered in the United States as of Tuesday, a notable increase from the early days of the outbreak. But that figure amounts to just around a quarter of the doses needed to give two to each of the roughly 1.6 million Americans estimated to be at high risk. Black and Hispanic men have received a disproportionately low number of doses, a gap that federal officials have recently moved more aggressively to erase.

Federal officials said on Wednesday that they were expanding eligibility for the vaccine with a preventive strategy known as pre-exposure prophylaxis, or PrEP, which they said would reach more people vulnerable to infection who had not yet had a known or presumed exposure.

The new research on the vaccine’s effectiveness came with substantial limitations. Changes in behavior, or differences in ages, health and testing, could have influenced the findings, the C.D.C. said.

Natalie Dean, a biostatistician at Emory University, said it was difficult for researchers to establish how exposed people were to monkeypox or their differences in behavior, particularly when dealing with delicate information.

“If you can refine it down to a population with a particular exposure, then it’s easier to compare a vaccinated group with a certain level of exposure and an unvaccinated group with the same level of exposure,” she said. She added that it could be challenging to interpret the results if people who got vaccinated were also those making more substantial changes to their behavior.

The study examined the results of just one dose of the two-dose vaccine, leaving federal scientists without a clear picture of the effectiveness of a complete series or its durability. Federal officials have said that fuller protection comes after receiving a second dose.

“We see some response after the first in the laboratory, but the really high responses that we want to really get that level 10 force field, as opposed to a level five force field, doesn’t happen until the second dose,” Dr. Demetre Daskalakis, the White House’s deputy monkeypox coordinator, said at the briefing. He added, “This just tells us to keep on trucking forward, because we need that second dose in arms.”

Dr. Walensky said her agency did not yet have data on the severity of monkeypox cases in vaccinated people or figures showing how many people who received a first dose came back for a second. Many of those at high risk have received just one dose, though in recent weeks more second doses were administered than first doses, according to C.D.C. data.

The research published on Wednesday also did not describe how people in the study were vaccinated — a critical piece of information needed to vet the federal government’s recent shift to a new method of vaccination. In August, the Food and Drug Administration began allowing providers to administer a lower dose of the shot in the skin layers, as opposed to the fat, stretching the vaccine supply at a time when doses were more limited. Many Americans in recent weeks have received the vaccine that way.

Dr. Walensky said at the briefing that “we’re going to need a little bit more time and a bit more numbers” to determine the effect of fractional dosing. The National Institutes of Health recently began a trial examining how well a full dose of the vaccine performs compared with fractional doses, an effort that will calculate antibody levels in trial participants who were vaccinated.

Scientists are still working to gain a basic understanding of the effectiveness of regular dosing of the vaccine, which was approved by federal regulators largely with animal data.

A recent study from Dutch researchers found that a single full dose of the shot generated low levels of antibodies against monkeypox, while two provided better but still modest protection. Federal officials have said they are discussing whether a three-dose regimen may be better.