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.

Gov. Andrew Cuomo wants New York state added to the list of countries requiring United Kingdom travelers to get tested before flying in the wake of a new variant of the novel coronavirus linked to an uptick in cases in England.

"If 120 countries require testing ... why are we doing nothing?" Cuomo said.

"I know and I believe my intuition is correct that it is another disaster waiting to happen,” Cuomo said.

British Airways and Delta have agreed to the governor’s request to test passengers flying from the U.K. to New York, Cuomo said. Virgin Atlantic is considering the request, Cuomo said.

A number of countries have introduced travel bans to the U.K. France, Germany and several other European countries have suspended flights, while similar measures have been taken around the world -- as far as Canada and Hong Kong.

Los Angeles officials arrested over 60 people involved in underground parties over the weekend.

In a statement, the Los Angeles County Sheriff’s Department said the parties were considered superspreader events, as they violated COVID-19 health protocols.

Around 10:45 p.m. Saturday, the Los Angeles Police Department responded to an underground party event in Los Angeles and found two people shot. One of the victims was pronounced dead at the scene, according to a statement from the LASD.

Then, at around 12:30 a.m. Sunday, officers responded to another underground party in Compton, where they arrested 67 people: 61 adults and six juveniles. One firearm was recovered.

A third underground party, located in Los Angeles, was shut down by the LAPD after a shooting took place.

"Sheriff Alex Villanueva has made it clear he will seek out and take law enforcement action against all underground party events occurring anywhere within Los Angeles County, who fall under the Health Orders of the County’s Department of Public Health," the LASD said in their statement. "The goal of these enforcement actions is to reduce the spread of COVID-19 and the risk to our vulnerable populations."