California Gov. Gavin Newsom urged residents to heed health warnings during the holiday season as the state's hospital system is pushed to the limit with COVID-19 patients.

He warned there could be multiple surges in cases "unless we are cautious and mindful about our travel plans this holiday season and about our proximity to people outside our households."

The number of the Intensive Care Unit patients is now 3,827, which was nearly double the 2,006 patients on Dec. 1, according to Newsom.

"On Monday, we were at 2.5% ICU availability and now as a state today, we are down to 1.1% ICU bed availability," he said.

"We had 8,517 people that were in our hospitals that tested positive for COVID-19 on Dec. 1st. Now 18,828 people are testing positive in just a three-week period," Newsom added.

-ABC News' Cammeron Parish contributed to this report

The new COVID-19 variant in the United Kingdom has likely infected tens of thousands of people, Jeffrey Barrett, director of the COVID-19 Genomics Initiative at the U.K. Wellcome Sanger Institute, said at a virtual Q&A hosted by the World Health Organization.

While there are just over 3,000 people who have the full genome sequence confirmed for the variant, only a small fraction of cases are sequenced, he said.

There’s no evidence the new variant is more deadly and it’s highly likely the vaccine will still work, officials said.

ABC News’ Sony Salzman contributed to this report.

Thursday, Dec. 24, Dr. Anthony Fauci’s 80th birthday, will be "Dr. Anthony S. Fauci" day in Washington, D.C., Mayor Muriel Bowser announced Wednesday.

"We are incredibly proud to count Dr. Fauci among the many DC residents who are sacrificing so much to keep our communities healthy and safe,” Bowser tweeted.

Hard-hit California reported 39,069 daily cases on Wednesday -- a 2% increase from Tuesday.

California’s daily cases are more than the daily cases in the United Kingdom, Russia, India, Italy and France, according to Johns Hopkins University data.

California is recording more than twice as many daily cases than the next hardest-hit states. Tennessee recorded 16,036 cases Tuesday and Texas had 16,607 cases, according to Johns Hopkins.

ABC News’ Bonnie Mclean contributed to this report.

The Centers for Disease Control and Prevention will now require a negative COVID-19 test for all air travelers coming to the United States from the United Kingdom in the wake of news about new strains of the virus being detected in the U.K.

Everyone arriving in the U.S. must have a negative PCR or antigen test from within 72 hours of departure, according to a statement from the CDC.

"The public health authorities in the United Kingdom recently announced the discovery of a new variant of SARS-CoV-2," the CDC wrote in its announcement. "Viruses constantly change through mutation, and preliminary analysis in the U.K. suggests that this new variant may be up to 70% more transmissible than previously circulating variants."

Passengers will have to provide written documentation of the negative test and airlines will be required to confirm the information, the CDC said. Airlines will have to refuse anyone without a negative test.

While the strain appears to be more transmissible, there's no evidence it is more deadly, or that the vaccines currently being rolled out won't be effective against it.

President Donald Trump, who is spending the Christmas holiday in Florida, will sign the executive order on Friday and it will go into effect on Monday.

Dozens of countries have largely banned travelers from the U.K., at least temporarily, including Canada, Mexico, Russia, China and India. The U.S. is one of a handful of countries that will require negative tests, including Greece and Spain.