President Joe Biden is expected to announce "as early as tomorrow" that the U.S. Centers for Disease Control and Prevention has updated its guidance for wearing masks outdoors, a source familiar with the discussions told NBC News.

The guidance will likely be for fully vaccinated people versus those who are not, the person said, cautioning that the final recommendations are still being finalized.

Dr. Anthony Fauci, the nation's top infectious disease expert, hinted Sunday that new guidance on the need for outdoor mask wearing would be coming "soon."

“I think it’s pretty common sense now that outdoor risk is really, really quite low,” said Fauci on ABC’s “This Week,” adding that vaccinated Americans particularly have a reduced risk.

A review paper published in The Journal of Infectious Diseases by researchers at the University of California, San Francisco, found that less than 10% of transmission occurs outdoors and the odds of spreading the virus indoors were 19 times higher, NBC News reports.

Some medical experts are increasingly calling for mask restrictions to be eased for outdoor activities.

Dr. Ashish Jha, dean of the Brown University School of Public Health in Providence, Rhode Island, told NBC News that wearing masks indoors should still be required, but said outdoor infections are rare and mostly tend to happen when large groups gather in confined spaces for long periods of time.

“At this point in the pandemic, with more than half of Americans vaccinated, it's pretty reasonable to start thinking about peeling back outdoor mask mandates," Jha said.

Dr. Paul Sax, clinical director of infectious diseases at Brigham and Women’s Hospital in Boston, Massachusetts, also agreed, writing in a blog post on the New England Journal of Medicine website that it might be time to lift the mandates, especially in places were people can safely distance.

“Transmissions do not take place between solitary individuals going for a walk, transiently passing each other on the street, a hiking trail, or a jogging track,” Sax wrote. “That biker who whizzes by without a mask poses no danger to us, at least from a respiratory virus perspective.”

Sax similarly stressed that indoor mask regulations should persist, at least until more people are vaccinated.

According to current CDC guidance, “masks may not be necessary when you are outside by yourself away from others, or with people who live in your household.”

However, experts at Northwestern University argue keeping masks on when you're outside — even after you're vaccinated — is not only a "social courtesy,” but also helps "model the behavior" for children, who can't yet get the shot. A mask shows “your civic engagement, the fact that you’re committed," said Mercedes Carnethon, vice chair of preventative medicine at Northwestern University Feinberg School of Medicine. "You’re sending a signal that you’re behaving in socially appropriate ways.”

Health experts hope the CDC will more clearly outline high-risk situations when masks are truly needed.

This is a developing news story. Check back for updates.