President-elect Joe Biden was administered the Pfizer vaccine on Monday afternoon at ChristianaCare Hospital in Delaware.

Dr. Jill Biden was vaccinated earlier on Monday.

Dr. Anthony Fauci is expected to get vaccinated Tuesday, according to the National Institutes of Health.

President Donald Trump has not been vaccinated because doctors wanted him to wait after he received monoclonal antibody treatment following his COVID-19 diagnoses, U.S. Surgeon General Jerome Adams said Monday.

"His doctors, who I've spoken with, wanted people to understand that if you have recently been given monoclonal antibodies, the FDA says you should wait several weeks before you get vaccinated,” Adams told reporters. “So the reason the president has not gotten vaccinated yet is because his doctors have advised him to wait."

ABC News' Ben Gittleson contributed to this report.

Injections of the newly authorized Moderna vaccine are now underway in the U.S.

In Boston, health care workers at the East Boston Neighborhood Health Center were among the first people to receive Moderna's vaccine. Dr. Jaime Gallegos, the first staffer to get the vaccine, held a sign that said, "Take that, COVID!"

The first cases containing the Moderna vaccine -- which was authorized by the FDA on Friday -- were seen Sunday morning being rolled out from a facility in Olive Branch, Mississippi, to be shipped to the FedEx hub in Memphis, Tennessee, for nationwide distribution.

The doses were packed into insulated coolers with specialized cold packs and a temperature monitor, according to pharmaceutical distribution company McKesson.

ABC News’ Arielle Mitropoulos contributed to this report.

The European Commission authorized the Pfizer vaccine on Monday, hours after the European Medicines Agency issued a recommendation granting a conditional marketing authorization (formal authorization) for the vaccine.

"It will be available to all EU countries at the same time, on the same conditions," tweeted EU Commission President Ursula von der Leyen.

French Prime Minister Jean Castex tweeted, "Vaccination will start in the 27 EU countries on December 27, 28 and 29."

ABC News’ Ibtissem Guenfoud contributed to this report.

The reproduction rate of a new COVID-19 variant linked to an uptick in cases in England is 0.4 higher than other known strains, Dr. Maria Van Kerkhove, the World Health Organization’s COVID-19 technical lead, said Monday.

That means the number of people an infected individual transmits to increases from 1.1 to 1.5 with the new variant.

There’s "zero evidence" that the new variant causes more severe disease, said Dr. Mike Ryan, executive director of the WHO’s Health Emergencies Programme.

The variant was reported to the WHO on Dec. 14. In the last few days, a number of countries have introduced travel bans to the United Kingdom.

The variant has also been identified in one patient in Australia, one in Iceland, one in Italy, one in the Netherlands, and believed to be in about 10 people in Denmark, Van Kerkhove said.

Studies around antibody response are underway. WHO officials said they expect results in the "coming days and the coming weeks." 

ABC News’ Christine Theodorou contributed to this report.