The Food and Drug Administration authorized use of Pfizer and partner BioNTech SE’s vaccine on Friday, according to an agency statement.

With the emergency clearance, the U.S. becomes the latest country to authorize the Pfizer-BioNTech vaccine. The U.K. raced ahead of the U.S. and European Union and cleared the vaccine on Dec. 2, and began immunizing older people and health-care workers this week. The shot is under review in the European Union.

The FDA has faced pressure from the White House for weeks to quickly authorize a vaccine, including as recently as Friday when White House Chief of Staff Mark Meadows told FDA Commissioner Stephen Hahn his job could be on the line if the agency didn’t get it done soon.

Following a Thursday vote by an advisory panel of outside experts backing the vaccine for use in people 16 and older, the FDA was weighing how to address reports of serious allergic reactions in the U.K., where the vaccine was approved earlier this month. Officials there have since told people with a history of significant allergic reactions not to get the vaccine. Marion Gruber, director of the FDA’s Office of Vaccines Research and Review, told the advisory panel Thursday that the FDA was seeking more information on the reactions from regulators in the U.K.

Pfizer’s vaccine, like one from Moderna Inc. that could also gain clearance next week, uses a new technology called messenger RNA that turns the body’s own cells into vaccine-producing factories to fight the coronavirus.

In a trial of almost 44,000 people, the Pfizer vaccine was 95% effective at preventing symptomatic cases of Covid-19, and protected people of all ages and ethnicities. It was able to prevent the most severe infections, according to clinical trial results.

Side effects in Pfizer’s clinical trial were mild and developed more frequently in younger participants in the study, specifically those aged 55 and younger, with no signs of serious safety concerns, according to an FDA staff report on the vaccine’s safety and efficacy. After the second injection, fatigue and headache were the most common systemic reactions, occurring in 59% and 52% of the younger participants, compared with 51% and 39% in those over age 56. Pfizer said patients in the trial did not experience serious allergic reactions.

It’s unclear whether the vaccine is effective at preventing Covid-19’s spread, according to the FDA staff report. It’s possible the shot didn’t work as well at preventing cases where people didn’t have symptoms, meaning there will likely be a continued need for basic public-health measures such as wearing masks and social distancing.

With initial supply very limited, the vaccine is expected to be given first to health-care workers and residents of long-term care facilities. Countries are clamoring for more doses, said Sean Marett, BioNTech’s chief business and commercial officer, in an interview.

“Everybody wants all their doses on January 2nd of next year, but that’s not practical,” he said in an interview. “This is a new technology, there does not exist a manufacturing network. We are building it.”

The shot requires special freezers to be kept extra cold temperatures until a few days before use, making distribution more complicated. Pfizer has developed special dry-ice containers to make it easier to store for facilities that don’t have the needed equipment.

Among the vaccine front-runners, the Pfizer-BioNTech shot is the only one that didn’t take funding from the White House-led Operation Warp Speed program to accelerate development, manufacturing and distribution of Covid-19 shots. However, the two companies have secured a $2 billion deal to supply 100 million doses to the U.S., with an option for 500 million more.