The Centers for Disease Control and Prevention on Thursday walked back contentious comments made in a TV interview by its director, Dr. Rochelle P. Walensky, suggesting that people 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 New York 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 her television interview, with Rachel Maddow on MSNBC, Dr. Walensky had 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 that they were sympathetic to Dr. Walensky and her obvious desire for Americans to continue to take precautions.
“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,” said John Moore, a virologist at Weill Cornell Medicine in New York. “It is an important public health message that needs to be gotten right.”
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 were also extremely effective at preventing infection.