(AP) — Pfizer and BioNTech say they’ve won permission Wednesday for emergency use of their COVID-19 vaccine in Britain, the world’s first coronavirus shot that’s backed by rigorous science — and a major step toward eventually ending the pandemic.

The move makes Britain one of the first countries to begin vaccinating its population as it tries to curb Europe’s deadliest COVID-19 outbreak.

Other countries aren’t far behind: The U.S. and the European Union also are vetting the Pfizer shot along with a similar vaccine made by competitor Moderna Inc.

Pfizer said it would immediately begin shipping limited supplies to the U.K. — and has been gearing up for even wider distribution if given a similar nod by the U.S. Food and Drug Administration, a decision expected as early as next week.

But doses everywhere are scarce, and initial supplies will be rationed until more is manufactured in the first several months of next year.

Pfizer CEO Albert Bourla called the U.K. decision “a historic moment.”

“We are focusing on moving with the same level of urgency to safely supply a high-quality vaccine around the world,” Bourla said in a statement.

While the U.K. has ordered enough Pfizer vaccine for 20 million people, it’s not clear how many will arrive by year’s end and adding to the distribution challenges is that it must be stored at ultra-cold temperatures.

Two doses three weeks apart are required for protection. First in line, the U.K. government says, are frontline health care workers and nursing home residents, followed by older adults.

British regulators also are considering another shot made by AstraZeneca and Oxford University. But Prime Minister Boris Johnson has warned “we must first navigate a hard winter” of restrictions to try to curb the virus until there’s enough vaccine to go around.

Every country has different rules for determining when an experimental vaccine is safe and effective enough to use. Intense political pressure to be the first to roll out a rigorously scientifically tested shot colored the race in the U.S. and Britain, even as researchers pledged to cut no corners. In contrast, China and Russia have offered different vaccinations to their citizens ahead of late-stage testing.

The shots made by U.S.-based Pfizer and its German partner BioNTech were tested in tens of thousands of people. And while that study isn’t complete, early results suggest the vaccine is 95% effective at preventing mild to severe COVID-19 disease. The companies told regulators that of the first 170 infections detected in study volunteers, only eight were among people who’d received the actual vaccine and the rest had gotten a dummy shot.

“This is an extraordinarily strong protection,” Dr. Ugur Sahin, BioNTech’s CEO, recently told The Associated Press.

The companies also reported no serious side effects, although vaccine recipients may experience temporary pain and flu-like reactions immediately after injections.

But experts caution that a vaccine cleared for emergency use is still experimental and the final testing must be completed. Still to be determined is whether the Pfizer-BioNTech shots protect against people spreading the coronavirus without showing symptoms. Another question is how long protection lasts.

The vaccine also has been tested in only a small number of children, none younger than 12, and there’s no information on its effects in pregnant women.