Coronavirus updates: Herd immunity by fall 'ambitious,' says surgeon general nominee

U.S. Surgeon General Jerome Adams resigned from his post Wednesday at the request of President-elect Joe Biden.

"I've been asked by the Biden team to step down as Surgeon General," Adams wrote on his official Twitter account.

In a lengthy statement that was posted on Facebook, Adams reflected on his role in the COVID-19 response.

"In the face of a once in a century pandemic, I sought to communicate the rapidly evolving science on this deadly adversary, and arm people with the knowledge and tools they needed to stay safe," he said. "I wasn’t always right -- because no one was, and this virus continues to humble all of us -- but I was always sincere in my efforts to speak to every day Americans, and address the terrible health inequities this virus exposed."

Biden has nominated former Surgeon General Vivek Murthy back to the position and as a senior adviser in the COVID-19 response. Murthy's nomination will need to be confirmed by the Senate.

Joe Biden plans to sign more than a dozen executive actions after he is sworn-in as the 46th president of the United States on Wednesday, including one that will impose a mask mandate in federal buildings and on federal land.

The new requirement will be part of Biden's "100 Days Mask Challenge," which asks Americans to wear face masks for that time period.

Biden plans to sign another executive order that will create the position of COVID-19 Response Coordinator and restore the National Security Council's Directorate for Global Health Security and Biodefense, which is responsible for pandemic preparedness and was dissolved by the Trump administration in 2018.

Biden also plans to reverse President Donald Trump's withdrawal from the World Health Organization.

ABC News' Justin Gomez and Molly Nagle contributed to this report.

Vatican City began offering free COVID-19 vaccinations to Rome's homeless community on Wednesday, according to Vatican spokesman Matteo Bruni.

The vaccinations took place in the atrium of the Paul VI Audience Hall, the massive auditorium where the Pope holds his weekly general audiences. An initial group of around 25 homeless individuals, who are all looked after in facilities run by the Office of Papal Charities, received their first doses of the vaccine Wednesday morning, according to Bruni.

"Further groups are to follow in the coming days," Bruni said in a statement.

Vatican City, an independent enclave surrounded by Rome that serves as the headquarters of the Roman Catholic Church, launched a COVID-19 immunization campaign last week, administering doses of a vaccine developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech. The tiny city-state has a population of only around 800 people but employs more than 4,000.

Both Pope Francis and his predecessor, Pope Emeritus Benedict XVI, have received their first doses of the vaccine.

The vaccination campaign is voluntary and people under the age of 18 are being excluded for the time being, according to Bruni.

Since the start of the pandemic, Vatican City has reported at least 27 confirmed cases of COVID-19, according to a real-time count kept by Johns Hopkins University.

ABC News' Phoebe Natanson contributed to this report.

Some hospitals in the United Kingdom look "like a war zone" as doctors and nurses workers grapple with an influx of COVID-19 patients, according to Patrick Vallance, the British government's chief scientific adviser.

"It may not look like it when you go for a walk in the park, but when you go into a hospital, this is very, very bad at the moment with enormous pressure and in some cases it looks like a war zone in terms of the things that people are having to deal with," Vallance told Sky News in an interview Wednesday.

He said there have been "huge numbers" of COVID-19 cases in recent days and that the country's health care system "is under enormous pressure at the moment." Official figures show nearly 38,000 people are currently hospitalized with COVID-19 across the U.K.

Vallance's comments come after the U.K. reported a record 1,610 additional deaths from COVID-19 on Tuesday, as the cumulative total approaches 100,000. Since the start of the pandemic, the country has confirmed more than 3.4 million cases of the disease, including more than 91,000 fatalities, according to the latest data published on the British government's website.

The U.K. -- -- an island nation of 66 million people made up of England, Scotland, Wales and Northern Ireland -- has the fifth-highest number of diagnosed cases worldwide and the fourth-highest death toll, according to a count kept by Johns Hopkins University.