• Fauci told CNBC that U.S. officials are concerned that they are seeing more breakthrough infections in fully vaccinated people.
• The more transmissible delta variant now makes up roughly 83% of sequenced Covid-19 cases in the country.
• Some areas of the U.S. are reimplementing mask mandates due to spikes in cases.

White House chief medical advisor Dr. Anthony Fauci said fully vaccinated people might want to consider wearing masks indoors as a precaution against the rapidly spreading delta variant in the U.S.

"If you want to go the extra mile of safety even though you're vaccinated when you're indoors, particularly in crowded places, you might want to consider wearing a mask," Fauci said in an interview Wednesday with CNBC.

Some areas of the U.S. are reimplementing mask mandates due to spikes in cases. The more transmissible delta variant now makes up roughly 83% of sequenced Covid-19 cases in the country, according to the Centers for Disease Control and Prevention.

"It's suggested that you wear a mask when you are indoors in a situation where you have a level of dynamics of virus in the community that's high," Fauci said.

He also said U.S. officials are concerned that they are seeing more breakthrough infections in fully vaccinated people in the U.S., even if they are more mild cases.

"That's something we obviously don't want to see," he said, noting that the delta variant was highly transmissible. "This virus is clearly different than the viruses and the variants that we've had experience with before. It has an extraordinary capability of transmitting from person to person."

Variants have increased in transmissibility from the original strain and some are proving to reduce the effectiveness of vaccines.

"Viruses don't mutate unless you allow them to replicate and spread in the community, you give them ample time and ample opportunity to mutate and you got a new variant," Fauci said. "The easiest and best and most effective way that we can prevent the emergence of a new variant and crush the already existing delta variant is to get everyone vaccinated."

In the U.S., 99.5% of Covid deaths are now among unvaccinated people. "That is a statistic that speaks for itself," Fauci said.

Despite the surge in new cases, Fauci said he doesn't believe U.S. officials will renew calls for a nationwide mask mandate "because there will be a lot of pushback on that."

Local counties and private businesses may choose to enforce mask mandates as the delta variant continues to spread in unvaccinated pockets of the country. Currently, almost two-thirds of counties in the U.S. have vaccinated less than 40% of their residents.

Colleges and universities have also taken the question of mandated vaccinations to court. Indiana University recently got the green light from a federal judge to require vaccines for students attending the fall semester.

Fauci said he doesn't see lockdowns making a comeback anytime soon.

"I don't see that on the horizon right now," Fauci said. "What I do see is increased testing and increased local mandates and a big push to get people vaccinated."