The Centers for Disease Control and Prevention responded Thursday to controversial comments from their director, Dr. Rochelle P. Walensky, back and suggested that people vaccinated against the coronavirus never get infected or pass the virus on to others.

The claim challenged the precautions the agency had urged vaccinated individuals to take just last month, such as wearing masks and collecting only in limited circumstances with unvaccinated individuals.

“DR. Walensky spoke broadly during this interview,” an agency spokesman told The Times. “It is possible that some people who are fully vaccinated will get Covid-19. The evidence is not clear as to whether they have the virus others can transfer. We continue to evaluate the evidence. “

The agency responded in part to criticism from scientists who found that current research is nowhere near enough to suggest that vaccinated people cannot spread the virus.

The data suggests that “vaccinated people are much harder to get infected, but don’t think for a second they can’t 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 responded to data released by the CDC showing that one dose of the Moderna or Pfizer BioNTech vaccine was 80 percent effective and two doses were 90 percent effective.

This certainly indicated that transmission from vaccinated people might be unlikely, but Dr. Walensky’s comments indicated that the protection was complete. “Our data from the CDC today suggests that vaccinated people do not carry the virus and do not get sick,” she said. “And not only in clinical studies, but also in real data.”

Dr. Walensky also stressed the importance of continuing to wear masks and taking precautions, including for people who have been vaccinated. However, the brief comment has been widely interpreted to mean that the vaccines provide complete protection against infection or transmission.

In a pandemic that regularly leads to scientific misunderstandings, experts said they sympathized with Dr. Walensky and her obvious desire that Americans continue to take precautionary measures. It was only Monday when she said the rising number of cases left her feeling “impending doom”.

“If Dr. Walensky had said that most people who were vaccinated don’t carry the virus, we wouldn’t be having this discussion,” said John Moore, a virologist at Weill Cornell Medicine in New York.

“What we do know is that the vaccines are very effective against infections – there is more and more data on them – but nothing is 100 percent,” he added. “It is an important public health message that needs to be properly understood.”

Updated

April 1, 2021, 6:59 p.m. ET

A misinterpretation could disrupt the agency’s urgent requests for vaccination, some experts said. By Wednesday, 30 percent of Americans had received at least one dose of vaccine and 17 percent were fully immunized.

“There can be no daylight between what the research shows – really impressive, but incomplete protection – and the description,” said Dr. Peter Bach, director of the Center for Health Policies and Results at Memorial Sloan Kettering Cancer Center in New York.

“This opens the door to the skeptics who believe the government is glossing over science,” said Dr. Bach, “completely undermines any remaining argument as to why people should continue to wear masks after vaccination.”

All coronavirus vaccines spectacularly prevent serious illness and death from Covid-19, but how well they prevent infection is less clear.

Clinical trials with the vaccines should only assess whether the vaccines prevent serious illness and death. The CDC’s research on Monday brought the welcome conclusion that the vaccines are also extremely effective at preventing infection.

3,950 healthcare workers, rescue workers and other people at high risk of infection took part in the study. Participants wiped their noses and sent in samples for testing each week, which allowed federal researchers to track any symptomatic or non-symptomatic infections. Two weeks after vaccination, the vast majority of people vaccinated remained virus-free, the study found.

Follow-up data from clinical studies support this finding. For example, in the results published Wednesday by Pfizer and BioNTech, 77 people who received the vaccine had coronavirus infection compared to 850 people who received a placebo.

“It is clear that some vaccinated people will be infected,” said Dr. Duprex. “We stop the symptoms, we keep people out of hospitals. But we’re not making them completely resistant to infection. “

The number of vaccinated people who become infected is likely to be higher among those who received Johnson & Johnson and AstraZeneca vaccines, which are less effective, experts say. (Still, these vaccines are worth taking because they consistently prevent serious illness and death.)

Infection rates can also be higher in people exposed to a variant of the virus that the immune system can bypass.

Cases across the country are on the upswing again and threaten a new upswing. Dr. Walensky’s comment came just a day after she emotionally urged the American public to continue taking precautionary measures.

“I ask you to hold on just a little longer to get the vaccine if you can, so that all of the people we all love will stay here when this pandemic ends,” she said.

With numbers soaring, it’s especially important that immunized people continue to protect those who have not yet been immunized against the virus, experts say.

“People who have been vaccinated shouldn’t throw away their masks at this point,” said Dr. Moors. “This pandemic is not over yet.”