Dr. Peter Hotez warned that the rapid spread of the Delta coronavirus variant has forced the threshold on the number of Americans who need vaccination to achieve herd immunity to be much higher.

“We are now realizing the variant that is so transmissible that we must have vaccinated 85%, maybe 90% of the country,” said Hotez, co-director of the Center for Vaccine Development at Texas Children’s Hospital.

“That’s not 85%, 90% of the adults, that’s 85-90% of the country, that is, all adults and all young people, if we really want to get through this, and that’s why he gives these extended timeframes before we get out of Covid-19 come out.”

White House senior medical advisor Dr. Anthony Fauci predicted the nation could have some control over Covid by spring 2022 if the “overwhelming majority of people who haven’t been vaccinated” are vaccinated. Covid deaths in the US are as high as in March at more than 1,000 a day.

US officials believe vaccination is the best way to tackle rising cases. According to the Centers for Disease Control and Prevention, about 51% of the entire US population will be fully vaccinated against Covid as of Sunday.

Hotez told CNBC’s “The News with Shepard Smith” that the US needs to “readjust” based on data from Israel, and this could include the need to wear masks outdoors.

“If you’re in cramped conditions outside, it’s pretty highly transferable, so certainly any type of sporting event, music event – you don’t want to go there without a mask,” Hotez said.

Oregon Governor Kate Brown signed a new statewide mandate on outdoor masks on Tuesday that requires people to wear masks in most outdoor environments regardless of vaccination status.