"So we would have people in isolation for a very long time if we were relying on PCRs," Walensky said.
Walensky also addressed Tuesday's news from the FDA that, according to early data, rapid antigen tests may be less sensitive when it comes to the omicron variant.
"We do know that the most sensitive test you can do is a PCR test," Walensky said. "So if you have symptoms and you have a negative antigen test, we do ask you to go and get a PCR to make sure those symptoms are not attributable to COVID."
Walensky said rapid tests do work "quite well," especially in places where people are being tested regularly, like at schools.
"They may not work as well as they have for the delta variant," Walensky said, but "we still are encouraging their use."