COVID-19 live updates: City sees fourfold increase in pediatric hospitalizations

About 73% of U.S. COVID-19 cases are now the omicron variant, but that number rises to 90% in areas like New York, CDC director Dr. Rochelle Walensky told "Good Morning America" Wednesday.

"Things are moving quickly," she said. "The doubling times of this virus are very fast, around two days."

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CDC director talks holiday guidance amid omicron surge

With a rise in cases due to the rapidly spreading omicron variant, Dr. Rochelle Walensky discusses if the CDC is planning on putting out any new guidance for holiday gatherings.

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But Walensky said the booster shot "will really help" with this variant.

"What we know about omicron is that it has a lot of mutations, and with more mutations we need more immune protection. And that's really why this booster shot will really help us," Walensky said.

Asked if President Joe Biden's decision to mail 500 million free at-home rapid tests to Americans in January is too late to help the current surge, Walensky responded, "We have been ramping up testing."

"We have much more testing now than we had just months ago," she said. "And we were in the middle of a delta surge as omicron hit, so really right now there are so many things that we can do in addition to testing to keep safe -- and that really does mean getting 40 million Americans who continue to be unvaccinated vaccinated and making sure that people get that booster shot."

Portugal said it would limit outdoor gatherings to 10 people and prohibit outdoor drinking on New Year's Eve. 

The country, which has 52 confirmed omicron cases, will also require most people to work from home starting on Saturday, the president's office told ABC News on Tuesday. 

The new restrictions, which are set to expire Jan. 10, will require people to present negative COVID-19 tests as they enter sporting events, theaters, weddings and other large gatherings. 

Bars, discos, and schools will close on Saturday, the president's office said. 

The country reported an uptick in COVID-19 cases between Dec. 16 and Dec. 19, but numbers were slightly down on Tuesday, with 2,752 newly diagnosed cases and 18 deaths, according to the World Health Organization's COVID-19 dashboard. Portugal's adult population is 87% fully vaccinated.

-ABC News' Aicha El-Hammar Castano

Chicago Mayor Lori Lightfoot announced a vaccine mandate for most indoor events as the city grasps with a rise in COVID-19 cases.

Starting Jan. 3, anyone over the age of 5 will have to show proof that they are fully vaccinated to "to dine indoors, visit gyms, or enjoy entertainment venues where food or drink are being served," the mayor's office said in a statement.

Patrons 16 and older will also need to "provide identification that matches their vaccination record," according to the statement.

Chicago is averaging more than 1,700 new COVID-19 cases a day -- a 79% increase from one week ago, the mayor's office said.

Elliot Hospital in New Hampshire is now seeing three to four times as many patients as it had at the beginning of the pandemic, Dr. Laura McPhee told ABC News.

She said their ICU is full with COVID-19 patients and staff is running thin.

"We're tired. It's been extremely hard on everybody," McPhee said, stressing that "most of the patients that we're seeing are unvaccinated."

"Most days I've been angry and frustrated. … Because this is preventable. It doesn't have to happen," she said. "I've not ever seen a patient here in the ICU who has been fully vaccinated with a booster."

-ABC News' Arielle Mitropoulos