By Jenny Strasburg and Stephen Fidler

The U.K. authorized a vaccine developed by Pfizer Inc. and Germany's BioNTech SE for emergency use, the first clearance by a Western government of one of three promising vaccines that have recently been shown to be effective against Covid-19.

Britain's Medicines and Healthcare products Regulatory Agency said the vaccine was safe to use. The government expects to start vaccinating people within days.

When will the vaccine be available in the U.K.?

Following Wednesday's announcement by Britain's Medicines and Healthcare products Regulatory Agency that the vaccine is safe to use. The vaccine program is being rolled out by the state-run National Health Service, which expects to start vaccinating people in the highest priority groups next week.

Why does Britain get the shot first?

The U.K. moved ahead of the U.S. and the European Union to designate the vaccine safe and effective for use, based on data from clinical trials that were updated for what regulators call "rolling reviews" as the trials progressed. The U.K. government accepted the advice of the country's medicines regulator and signed off on use of the vaccine. Other regulators also have been looking at data as trials progress. The U.S. has scheduled a public hearing in December to discuss the Pfizer-BioNTech vaccine; the EU medicines regulator also will hold an online public hearing in December about various Covid-19 vaccines.

When does it become available in the U.S.?

It may not be too long. A panel of experts advising the Food and Drug Administration is planning to review the vaccine on Dec. 10. The FDA could authorize it soon after, and supplies should become available not long after that. In fact, Pfizer has already started shipping shots to distribution sites.

How many doses will the U.K. have for distribution?

The U.K. has secured access to 40 million doses of the Pfizer-BioNTech vaccine, enough for 20 million people because it is a two-shot vaccine. The government in early November said it expects to receive around 10 million doses this year, with the rest to follow next year.

U.K. Health Secretary Matt Hancock said Wednesday the country is expecting an initial 800,000 doses to arrive in Britain next week. He said the speed at which vaccinations will take place will depend on how quickly Pfizer can manufacture the shot at a plant in Belgium, but the government is expecting many millions of doses by the end of the year.

"This will start small and ramp up. The vast majority of vaccinations we expect to be in the new year," Mr. Hancock said in an interview with the British Broadcasting Corp.

Who gets the first shots in the U.K.?

The highest priority group to get the vaccine are people in nursing homes. But the vaccine comes in packs of 975 doses and the regulators haven't yet given approval to split them up into smaller lots that can be taken to nursing homes. That means people who get the first vaccines therefore will be the over-80s and other high-priority groups who can travel to 50 hospital hubs around the country.

Overall, people will be prioritized in nine tiers according to an independent scientific and medical assessment of their risk of dying from Covid-19.

The first in line will be staff and the more than 400,000 residents in nursing homes followed by health workers and the estimated 3.4 million people over 80. People will then be given priority according to age in decreasing five-year intervals -- down to those aged 50 and over.

Nobody under 50 is on the list, except for people who are clinically extremely vulnerable and who will be vaccinated along with the over-70s, and those over 16 in at-risk groups -- such as people with diabetes, morbid obesity, or a number of chronic conditions. These people are in line to receive shots after the over-65s.

What about the vaccine from Moderna and the one from the University of Oxford and AstraZeneca?

They might not be far behind, though in different countries. Moderna Inc. on Monday said it would ask European and U.S. regulators to authorize use of its vaccine. Last week, the U.K. government asked the British medicines regulator to review the vaccine developed by AstraZeneca PLC and the University of Oxford for emergency authorization in the U.K. Both companies have signaled that their vaccines could be available for use before year-end. The British minister for vaccine rollout, Nadhim Zahawi, said Monday he expected that the Moderna version would, subject to authorization, be distributed in the spring.

Write to Jenny Strasburg at jenny.strasburg@wsj.com and Stephen Fidler at stephen.fidler@wsj.com

(END) Dow Jones Newswires

12-02-20 1511ET