CDC on Tuesday issued new guidance on when it's safe—and when it isn't—for fully vaccinated Americans to exercise, socialize, and gather outdoors without wearing masks, a move the agency said "may help improve coronavirus vaccine acceptance and uptake."

Details on the new guidance

In the guidance, CDC said all Americans, whether vaccinated or not, can be maskless outside if they are:

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• Walking, running, or biking alone or with members of their household; or
• Attending a small, outdoor gathering with family and friends who are fully vaccinated.

In addition, according to the guidance, fully vaccinated Americans can be maskless outdoors if they are:

• Attending a small, outdoor gathering with a mix of fully vaccinated and unvaccinated people; or
• Eating at an outdoor restaurant with friends from multiple households.

However, CDC said both fully vaccinated and unvaccinated Americans need to wear masks while attending crowded outdoor events, such as a live show or a parade, and during all indoor activities.

CDC also released a detailed, visual guide to the occasions when both vaccinated and unvaccinated Americans need to wear masks—including guidance on activities such as attending full-capacity worship services, going to movie theaters, and eating in indoor restaurants.

"Taking steps toward relaxing certain measures for vaccinated people may help improve coronavirus vaccine acceptance and uptake," the guidance said. "Therefore, there are several activities that fully vaccinated people can resume now, at low risk to themselves, while being mindful of the potential risk of transmitting the disease to others."

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The examples presented in the guidance "show that when you are fully vaccinated, you can return to many activities safely … and begin to get back to normal," CDC Director Rochelle Walensky said. "And the more people who are vaccinated, the more steps we can take toward spending time with people we love, doing the things we love to enjoy. I hope this message is encouraging for you. It shows just how powerful these vaccines are."

Walensky added that the United States is seeing a "hopeful decline" in Covid-19 cases that could be due to the nationwide vaccine rollout.

"Each day, more and more Americans are rolling up their sleeves and getting vaccinated and likely contributing to these very positive trends," she said.

President Joe Biden said the guidance is a landmark moment in the Covid-19 epidemic, adding that scientists are convinced the risk of transmitting or contracting the new coronavirus outdoors is "very, very low," especially if the person is vaccinated.

"The bottom line is clear: If you're vaccinated, you can do more things, more safely, both outdoors as well as indoors," Biden said. "So for those who haven't gotten their vaccination yet, especially if you're younger or thinking you don't need it, this is another great reason to go get vaccinated—now."

What experts are saying about CDC's guidance

Monica Gandhi, an infectious disease expert at the University of California-San Francisco, praised CDC's new guidance as aligning with evidence that the risk of outdoor coronavirus transmission is low.

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She added that the new guidance is "a great strategy to encourage those who are on the fence to get vaccinated."

"Public health messaging since the time of HIV that focuses on positive, rather than negative, reinforcement has been shown to be more effective, so the CDC guidelines that vaccinated people don't have to mask outdoors will hopefully help persuade some of the vaccine hesitant in the U.S. to get the vaccine," Gandhi said.

Similarly, Amanda Castel, an epidemiology professor at George Washington University, said the new guidance is "laying the foundation for, 'This is where we could go if we're able to get everyone on board and vaccinated.'"

But Linsey Marr, an expert in aerosol science at Virginia Tech, warned that the granularity of the new guidance, with its highly specific guidance of what behavior is and is not acceptable, could be confusing to the public.

"I can't remember this. I would have to carry around a sheet of paper—a cheat sheet with all these different stipulations," she said. "I worry that this is not as helpful as it could be" (Rabin et. al., New York Times, 4/27; Joseph, STAT News, 4/27; Sun, Washington Post, 4/27; Watson/Quinn, CBS News, 4/27; Coleman, The Hill, 4/27; New York Times, 4/27; Owens, "Vitals," Axios, 4/28; Courage, Vox, 4/27).