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.