Fully vaccinated Americans may "want to consider" masks indoors, Dr. Anthony Fauci said.
He said the Delta variant had an "extraordinary capability of transmitting from person to person."
More fully vaccinated Americans are getting COVID-19, albeit with milder, or no, symptoms.
Dr. Anthony Fauci, the US's top infectious-disease expert, said fully vaccinated people might "want to consider" wearing masks indoors to help protect against the Delta coronavirus variant, which is spreading fast across the country.
Fauci, who is President Joe Biden's chief medical advisor, told CNBC on Wednesday that "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 wearing a mask indoors was "suggested" when a virus was spreading fast in a community.
The Delta variant is more infectious and has mutations that help it avoid the body's immune response. As it spreads in the US, more fully vaccinated Americans are catching COVID-19 - albeit with milder, or no, symptoms.
"This virus is clearly different than the viruses and the variants that we've had experience with before," Fauci said. "It has an extraordinary capability of transmitting from person to person."
The Centers for Disease for Disease Control and Prevention recommends that fully vaccinated people don't need to wear masks, except in certain situations, for example during flights.
States, workplaces, and businesses can enforce their own mask rules.
The public-health body has come under pressure to review its guidance in recent weeks because of the emergence of the Delta variant, which is thought to be at least 50% more infectious than the formerly dominant Alpha variant.
Dr. Jerome Adams, the former surgeon general, said Sunday that "the emerging data suggests CDC should be advising to vax it and mask it." The World Health Organization on June 26 said fully vaccinated people should wear masks in public settings.
More than 161 million Americans are fully vaccinated, or 48% of the population, according to the CDC.
Some states have already urged fully vaccinated residents to put their masks back on.
The Los Angeles County Department of Public Health on June 28 said it "strongly recommends everyone, regardless of vaccination status, wear masks indoors in public places as a precautionary measure," citing the Delta variant.
Nevada's Clark County, home to Las Vegas, reintroduced mask mandates indoors for all workers on Wednesday.
Despite his views on masks, 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."
Real-world UK data published Wednesday found the Pfizer-BioNTech COVID-19 vaccine was 88% effective at preventing COVID-19 with symptoms caused by the Delta variant.
In the US, data has indicated that more than 99.5% of COVID-19 deaths are among unvaccinated people. "That is a statistic that speaks for itself," Fauci told CNBC.
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