CDC officials made a breakthrough decision on masks. This is what convinced them.

The Centers for Disease Control and Prevention on Thursday said masks and social distancing are no longer necessary for people who have been fully vaccinated against Covid-19. It's a move the agency said was driven by scientific evidence that the vaccines play a major role in curbing both infections and transmission of the virus.

In announcing the agency's updated guidelines, CDC Director Dr. Rochelle Walensky said there are "numerous reports in the literature" to demonstrate the safety and real-world effectiveness of Covid-19 vaccines.

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Walensky highlighted, in particular, three recent studies that demonstrated the impact of the vaccines on symptomatic and asymptomatic infections and one study published just last week on the effectiveness of Covid-19 vaccines against two variants that are known to be circulating in the United States.

The findings all add to a growing body of evidence that the vaccines are effective at preventing severe illness and death from Covid-19 and that they help prevent people from spreading the virus to others.

"The trends are all going down because vaccines are making a big difference," said Dr. Isaac Weisfuse, a medical epidemiologist at Cornell University and former New York City deputy health commissioner. "The fewer people you have who are susceptible, the more likely the trends will keep going down."

In the U.S., the number of newly reported Covid-19 cases, hospitalizations and deaths are all decreasing. The most recent seven-day average for new cases fell about 23 percent over the previous week, Walensky said. The seven-day average for daily deaths also declined to 587 per day, according to the CDC.

"Today, Covid-19 deaths are at the lowest point since April 2020," Andy Slavitt, the White House Covid-19 adviser, said Thursday in a news briefing.

These statistics help reinforce that the vaccines are working — and working well, said Dr. Monica Gandhi, an infectious disease physician and professor of medicine at the University of California, San Francisco.

"It's like we reached a tipping point in terms of the weight of the evidence showing that these are profoundly effective vaccines, beyond our wildest dreams, and they're really good at blocking transmission," Gandhi said.

The change in recommendations was overdue, according to Dr. David Dowdy, an associate professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health.

"I think part of the problem before this was that there was something of a mixed message: The vaccines are very effective, but you still have to wear a mask," Dowdy said. "This is now a strong statement that we know these vaccines work, and for those who are fully vaccinated, it's appropriate to take some steps toward living life a bit more normally."

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In one of the studies cited by the CDC, the Pfizer-BioNTech vaccine was shown to be 97 percent effective at protecting against symptomatic infection and 86 percent effective at protecting against asymptomatic infection. Those results, published May 6 in the Journal of the American Medical Association, were based on a study of more than 6,700 vaccinated health care workers in Israel.

Walensky also referenced two recent U.S.-based studies that were published in the CDC's Morbidity and Mortality Weekly Report. One study demonstrated that Covid-19 vaccines were 90 percent effective at preventing both symptomatic and asymptomatic infection among nearly 4,000 health care workers and front-line workers. A second study found the Pfizer and Moderna vaccines were 94 percent effective at preventing Covid-19-related-hospitalizations among adults 65 and older who have been fully vaccinated.

The CDC's study on adults 65 and up was particularly important, Gandhi said, because there were early concerns about how well the vaccines would perform in older populations.

"The cumulative weight of all these studies show that taking masks off a vaccinated person is completely fine," she said. 

While it is possible for someone who is fully vaccinated to get infected, these breakthrough infections are considered very rare. Out of the more than 117 million people in the U.S. who have been fully vaccinated, just 9,245 people later tested positive for Covid-19. The CDC has also said illnesses from breakthrough infections are typically mild.

Walensky also highlighted the results of a study published May 5 in The New England Journal of Medicine that looked at the effectiveness of the Pfizer vaccine against two coronavirus variants. The research, based on the results of a mass vaccination campaign in Qatar, showed the vaccine was 89.5 percent effective at preventing infection from the so-called B.1.1.7 variant, a more-contagious strain of the virus that was first reported in the U.K. The Pfizer vaccine was also 75 percent effective at protecting against the B.1.351 variant, which was first identified in South Africa.

These results are significant because there have been concerns that vaccines wouldn't be as effective against the B.1.351 variant, and the B.1.1.7 variant has taken hold in the U.S., becoming, in early April, the dominant strain in the country.

It's possible other variants of the virus could emerge that force the CDC to alter its guidance, but Dowdy said it's the right time to ease restrictions for people who are fully vaccinated.

"If and when those variants emerge, we will react accordingly," he said. "But the important message right now in the U.S. is that things are trending in the right direction, to a place where we're able to recommend to over one-third of Americans that they can now take off their masks."

Weisfuse said the updated guidance represents an important milestone for the country and a breakthrough in the course of the pandemic.

"It has been such a long and terrible road," he said, "but this is a landmark day."

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