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.

Dr. Anthony Fauci told The Washington Post that his message for Christmas is to "tone things down," including gathering outdoors when possible, wearing a mask when not eating or drinking, and keeping gatherings to as few people as possible and within your household.

Fauci is not only celebrating Christmas on Thursday, but also his 80th birthday. And for the first time, Fauci won’t be with his daughters for the big day.

"This is the first time since the birth of my daughters, and they are now in their late 20s and early 30s, that we have not been together on Christmas Eve,” Fauci said. “I'm not alone in this feeling that kind of pain of separation from our loved ones.”

His milestone birthday was marked with a surprise Zoom party organized by his wife.

"I want to practice what I preach. I don't want to tell the country to do something that I'm not willing to do myself,” Fauci said. “I was just on a Zoom call with the girls ... I think I lost about a liter of fluid in tears, to be honest, with hearing their expressions of love and concern to me. But we ended it on a happy note because we know that sooner or later, very likely sooner, as we get into 2021 and things get better and vaccines help us, that this time next year we'll be back again with the Fauci family celebrating.”

ABC News’ Stephanie Ebbs contributed to this report.