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Researchers pushed back after the C.D.C. director asserted that vaccinated people “do not carry the virus.”
The Centers for Disease Control and Prevention on Thursday walked back controversial comments made by its director, Dr. Rochelle P. Walensky, suggesting that people who are vaccinated against the coronavirus never become infected or transmit the virus to others.
The assertion called into question the precautions that the agency had urged vaccinated people to take just last month, like wearing masks and gathering only under limited circumstances with unvaccinated people.
“Dr. Walensky spoke broadly during this interview,” an agency spokesman told The Times. “It’s possible that some people who are fully vaccinated could get Covid-19. The evidence isn’t clear whether they can spread the virus to others. We are continuing to evaluate the evidence.”
The agency was responding in part to criticism from scientists who noted that current research was far from sufficient to claim that vaccinated people cannot spread the virus.
The data suggest that “it’s much harder for vaccinated people to get infected, but don’t think for one second that they cannot get infected,” said Paul Duprex, director of the Center for Vaccine Research at the University of Pittsburgh.
In a television interview with MSNBC’s Rachel Maddow, Dr. Walensky referred to data published by the C.D.C. showing that one dose of the Moderna or Pfizer-BioNTech vaccine was 80 percent effective at preventing infection, and two doses were 90 percent effective.
That certainly suggested that transmission from vaccinated people might be unlikely, but Dr. Walensky’s comments hinted that protection was complete. “Our data from the C.D.C. today suggests that vaccinated people do not carry the virus, don’t get sick,” she said. “And that it’s not just in the clinical trials, it’s also in real-world data.”
Dr. Walensky went on to emphasize the importance of continuing to wear masks and maintain precautions, even for vaccinated people. Still, the brief comment was widely interpreted as saying that the vaccines offered complete protection against infection or transmission.
In a pandemic that regularly spawns scientific misunderstanding, experts said they were sympathetic to Dr. Walensky and her obvious desire for Americans to continue to take precautions. It was only Monday that she said rising caseloads had left her with a sense of “impending doom.”
“If Dr. Walensky had said most vaccinated people do not carry virus, we would not be having this discussion,” said John Moore, a virologist at Weill Cornell Medicine in New York.
“What we know is the vaccines are very substantially effective against infection — there’s more and more data on that — but nothing is 100 percent,” he added. “It is an important public health message that needs to be gotten right.”
Misinterpretation could disrupt the agency’s urgent pleas for immunization, some experts said. As of Wednesday, 30 percent of Americans had received at least one dose of a vaccine and 17 percent were fully immunized.
“There cannot be any daylight between what the research shows — really impressive but incomplete protection — and how it is described,” said Dr. Peter Bach, director of the Center for Health Policy and Outcomes at Memorial Sloan Kettering Cancer Center in New York.
“This opens the door to the skeptics who think the government is sugarcoating the science,” Dr. Bach said, “and completely undermines any remaining argument why people should keep wearing masks after being vaccinated.”
All of the coronavirus vaccines are spectacularly successful at preventing serious disease and death from Covid-19, but how well they prevent infection has been less clear.
Clinical trials of the vaccines were designed only to assess whether the vaccines prevent serious illness and death. The research from the C.D.C. on Monday brought the welcome conclusion that the vaccines are also extremely effective at preventing infection.
The study enrolled 3,950 health care workers, emergency responders and others at high risk of infection. The participants swabbed their noses each week and sent the samples in for testing, which allowed federal researchers to track all infections, symptomatic or not. Two weeks after vaccination, the vast majority of vaccinated people remained virus-free, the study found.
Follow-up data from clinical trials support that finding. In results released by Pfizer and BioNTech on Wednesday, for example, 77 people who received the vaccine had a coronavirus infection, compared with 850 people who got a placebo.
“Clearly, some vaccinated people do get infected,” Dr. Duprex said. “We’re stopping symptoms, we’re keeping people out of hospitals. But we’re not making them completely resistant to an infection.”
The number of vaccinated people who become infected is likely to be higher among those receiving vaccines made by Johnson & Johnson and AstraZeneca, which have a lower efficacy, experts said. (Still, those vaccines are worth taking, because they uniformly prevent serious illness and death.)
Infection rates may also be higher among people exposed to a virus variant that can sidestep the immune system.
Cases across the country are once again on the upswing, threatening a new surge. Dr. Walensky’s comment came just a day after she made an emotional appeal to the American public to continue taking precautions.
“I am asking you to just hold on a little longer, to get vaccinated when you can, so that all of those people that we all love will still be here when this pandemic ends,” she said.
Given the rising numbers, it’s especially important that immunized people continue to protect those who have not yet been immunized against the virus, experts said.
“Vaccinated people should not be throwing away their masks at this point,” Dr. Moore said. “This pandemic is not over.”