The New York City Ballet has canceled the rest of its performances of "The Nutcracker" due to COVID-19 cases, ABC New York station WABC reported.

"The Nutcracker" was set to run through Sunday.

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. Anthony Fauci strongly recommended against large New Year's Eve parties at Wednesday's White House briefing.

"Should you change or cancel your plans? If your plans are to go to a 40-50 person New Year's Eve party with all the bells and whistles and everybody hugging and kissing and wishing each other a happy new year, I would strongly recommend that this year, we do not do that," Fauci said.

Fauci said "all indications point to a lesser severity of omicron versus delta." But he warned, "we should not become complacent since our hospital system could still be stressed in certain areas."

White House COVID-19 coordinator Jeff Zients said, "We have directly helped more than 30 states and territories by deploying over 2,100 federal personnel and thousands of ambulances, ventilators and other critical supplies."

Supplies include gloves, masks, respirators and face shields, he said.

He added, "More than 13,000 National Guard members have been activated in 48 states to support the COVID response, from vaccinations, to testing, to clinical care."

At the briefing, CDC Director Rochelle Walensky defended the agency's shortened isolation guidelines.

"We do know the vast majority of viral transmission happens in those first five days, somewhere in the 85 to 90% range. So if a person can isolate for the first five days they absolutely should," she said. "We also don't know that antigen tests give a good indication of transmissibility at this stage of infection. On the other hand, we know that after five days people are much less likely to transmit the virus and that masking further reduces that risk. And this is why people need to mask for five days after the five days of isolation."

After five days, asymptomatic people with COVID-19 can leave isolation, but must wear masks around others, according to the new guidelines.

-ABC News' Cheyenne Haslett