White House senior medical advisor Dr. Anthony Fauci said people who are fully vaccinated should consider wearing masks indoors as a precaution against the rapidly spreading Delta variant in the US

“If you want to walk that extra mile of safety indoors, especially in crowded places, despite being vaccinated, consider wearing a mask,” Fauci said in an interview with CNBC on Wednesday.

Some areas of the United States are reintroducing mask requirements due to spikes in cases. The more transmissible Delta variant now accounts for around 83% of the sequenced Covid-19 cases in the country, according to the Centers for Disease Control and Prevention.

“It is recommended that you wear a mask if you find yourself in an indoor situation where virus dynamics are high in the community,” Fauci said.

Fauci also said US officials are concerned they are seeing more breakthrough infections in fully vaccinated people in the US, even if they are milder cases.

“Of course we don’t want to see that,” he said, noting that the Delta variant is highly transferable. “This virus is very different from the viruses and variants that we have previously experienced. It has an exceptional ability to transmit from person to person.”

The portability of variants from the original strain has increased, and some have been shown to decrease the effectiveness of vaccines.

“Viruses don’t mutate unless you allow them to replicate and spread through the community. You give them ample time and opportunity to mutate and you have a new variant,” Fauci said. “The easiest, best and most effective way to prevent the emergence of a new variant and destroy the existing Delta variant is to have everyone vaccinated.”

In the United States, 99.5% of Covid deaths are now among unvaccinated people. “This is a statistic that speaks for itself,” said Fauci.

Despite the surge in new cases, Fauci doesn’t believe US officials will renew calls for a nationwide mask mandate “because there will be a lot of headwinds.”

Local counties and private companies may choose to enforce mask requirements as the delta variant spreads more widely in unvaccinated areas of the country. Currently, nearly two-thirds of counties in the United States have vaccinated less than 40% of their residents.

Colleges and universities have also brought the question of mandatory vaccinations to court. Indiana University recently got the go-ahead from a federal judge to require vaccines for college students entering the fall semester.

Fauci said he doesn’t see any comeback from lockdowns anytime soon.

“I don’t see that on the horizon right now,” said Fauci. “What I’m seeing is more tests and more local mandates and a lot of pressure to get people vaccinated.”