COVID updates: Secretary of Defense Lloyd Austin tests positive

The newly updated CDC guidelines don't require testing at the end of isolation because PCR tests can stay positive for up to 12 weeks, CDC Director Dr. Rochelle Walensky told "Good Morning America" Wednesday.

"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."

New COVID-19 cases increased 11% last week over the week prior, The World Health Organization said on Tuesday. 

Almost 5 million new infections were recorded in the week that ended Dec. 26, the agency said in its weekly epidemiological update. The rise followed a "gradual increase" since October, the agency said. 

WHO officials said the risk from the omicron variant "remains very high."

The agency said early data from the United Kingdom, South Africa and Denmark suggested a reduced risk of hospitalization for those infected with omicron, but said "further data are needed to understand the clinical markers of severity including the use of oxygen, mechanical ventilation and death, and how severity may be impacted by vaccination and/or prior SARSCoV-2 infection."

San Francisco officials canceled the city's New Year's Eve fireworks show, citing concerns about the spread of the omicron variant.  

"After closely monitoring local health indicators, the decision to cancel is a proactive measure that will best protect SF & essential front-line workers," the city's department of emergency management said in a statement on Tuesday. 

About 84% of eligible residents are fully vaccinated and 55% have had a booster shot, according to city health data. But omicron still "poses a significant risk," officials said in a statement. 

“While we are all understandably eager to ring in a new year with San Francisco’s customary New Year’s Eve fireworks show, we must remain vigilant in doing all we can to stop the spread of the COVID-19 Omicron variant,” Mayor London Breed said.

Dr. Jerome Adams, the U.S. surgeon general under former President Donald Trump, slammed the U.S. Centers for Disease Control and Prevention over its change on isolation times for infected individuals.

In a Twitter thread, Adams criticized the agency's decision to omit the recommendation for COVID-19-positive individuals to take a COVID-19 test, prior to ending isolation.

The CDC reduced the isolation period from 10 days to five for asymptotic patients.

"Regardless of what CDC says, you really should try to obtain an antigen test (I know- easier said than done) and confirm it’s negative prior to leaving isolation and quarantine. There’s not a scientist or doctor I’ve met yet who wouldn’t do this for themselves/ their family," Adams tweeted.

The former surgeon general added, "This isn’t about the best science – it’s a compromise to keep the economy open in the face of inadequate tests."

-ABC News' Arielle Mitropoulos