The U.K. green light on Wednesday punctuates a monthslong sprint by the two drugmakers, which  teamed up earlier this year  and then pulled ahead of two other Western pharmaceutical giants, each with its own promising shot. Vaccines typically take years to bring to market.

It also marks a key milestone in efforts to translate a promising new vaccine technology into a widely available shot. It was developed, tested, authorized and is now poised to be distributed amid a pandemic that has sickened tens of millions of people and killed  more than 1.4 million around the world .

The U.K. has ordered 40 million doses, enough to vaccinate 20 million people. 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 the shot can be manufactured 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.

People familiar with the matter said the U.K. might get four million to five million doses this year, but the number is in flux and will depend on production and other potential regulatory authorizations. The U.K. said it planned to use that limited supply to start vaccinating nursing-home residents and staff, and then move on to other highly vulnerable groups.

That sets up the U.K. as a potential test case for how distribution—and uptake—of the vaccine might unfold in other countries. The National Health Service, Britain’s state-funded health system, runs hospitals across the country, and has been tapped to distribute the shots. The U.K. has also long been a battleground for vaccine skeptics, setting up a potential challenge here for the government: persuading people to get the shot.
A panel of experts advising the Food and Drug Administration is planning to review the vaccine on Dec. 10. The FDA could authorize soon after, and supplies should become available not long after that. Pfizer has  already started shipping shots  to distribution sites.

Pfizer and BioNTech joined forces at the onset of the virus outbreak and edged ahead of two  other promising vaccine candidates , one from  Moderna  Inc. and the other developed by the University of Oxford and  AstraZeneca  PLC.

Moderna’s vaccine is being  reviewed by the FDA as well  and could be authorized for distribution in limited quantities in the U.S. by the end of the year. China and Russia have been vaccinating their populations with homegrown vaccines, too. The U.S., Europe and several other developed countries have held out hope for one or all of the three leading Western shots.

Results of clinical trials for each, disclosed over recent months, have raised hope that governments can quickly start inoculating their populations to build widespread immunity and begin to more confidently reopen businesses, schools and tourist destinations that have been closed, opened and then closed again amid the ebb and flow of the virus. When Pfizer and BioNTech first disclosed early, promising results,  global stock markets shot higher .

Pfizer said last month that trials found its vaccine to be 95% effective in preventing symptomatic Covid-19. Moderna days later said its vaccine, developed with the same genetic technology upon which the Pfizer vaccine is based,  showed efficacy of 94.5% .

Both vaccines  use a new technology , leveraging mRNA, short for the molecular couriers called messenger RNA that carry genetic instructions to cells. The shots deliver mRNA that prompts cells to make a synthetic version of the spike protein that juts from the surface of the new coronavirus. That protein triggers the immune system to defend against the virus.

The shot developed by Oxford and AstraZeneca works like a more traditional vaccine, introducing a weakened virus, in this case one found in chimpanzees, into the body to trigger immunity. It showed  an efficacy range of 62% to 90%, depending on the dosage .

The U.K. has  ordered its medicines regulator to review  that vaccine for emergency use, too. AstraZeneca is still conducting trials in the U.S. and is expected to submit its vaccine to the FDA once it has enough data.

The European Medicines Agency, the top medical regulator for the European Union, has said it hopes to authorize at least one of the three vaccines by the end of the year.

Russia, which was criticized in the West over the speed of development, has claimed its vaccine has efficacy of more than 90%. Chinese developers haven’t provided data from the final clinical trials of their shots.

The U.K.’s emergence as the first Western country to get a vaccine is the result of fast work by the country’s Medicines and Healthcare products Regulatory Agency. Britain is transitioning out of the EU, with the exit fully kicking in Jan. 1. As a member of the bloc, authorization of the U.K.’s new drugs were in the past overseen by the EMA, which had been based in London until Brexit prompted it to move to Amsterdam.

The MHRA in the past worked in close partnership with the EMA, handling a significant portion of the Europe-wide portfolio of medicines and medical devices awaiting authorization. As such, its staff of 1,320 is experienced in the kind of safety scrutiny the vaccines have attracted.

Brexit has meant the work the MHRA did for all of Europe was reduced significantly, freeing up knowledgeable staff to move quickly when vaccine data became available, according to researchers familiar with the agency.

While drug authorization for British citizens is still in most cases in the hands of the EMA until the end of the year, the U.K. government used a long-held power to authorize a drug for emergency use, after the MHRA signed off on its safety.

People who have worked with the U.K. agency in recent months say it has been more proactive in cooperating with drug companies as they provide data, asking additional questions and requesting more information that could speed a review. Like the FDA and the EMA, the U.K. regulator has received data on a rolling basis. Recently, the MHRA’s rolling review has been a “much more interactive experience,” said Sarah Blagden, associate professor of experimental cancer therapeutics at the University of Oxford, who oversees cancer trials. “The regulators become part of the discussion very, very early on,” she added.

June Raine, head of the MHRA, said the agency followed international standards for reviewing vaccines, starting in June with the setting of effectiveness hurdles and carrying through to the testing of manufactured doses for quality in recent days. Asked during a press briefing about public concerns that the regulator rushed the process, Dr. Raine said, “Everyone can be absolutely confident that no corners whatsoever have been cut.”

The U.K. government has been among the world’s most proactive in buying vaccines. It has now secured access to roughly 357 million doses from seven manufacturers. Given the current vaccines are expected to require two shots, that is enough to vaccinate the population 2.7 times over, providing some leeway if not all vaccines come to fruition.

he rapid authorization could provide a political boost to Prime Minister Boris Johnson, whose handling of the pandemic so far has been criticized from both sides of the political spectrum in the U.K. From his own party, lawmakers have argued his lockdowns have been too strict, while opposition parties say he was too slow to appreciate the threat of the coronavirus when it emerged. They point to expensive programs, such as a countrywide test-and-trace system, that have struggled to make an impact on infection rates.

The U.K. has suffered the fourth-highest death rate of the major European countries—after Belgium, Spain and Italy—and its economic performance has been among the weakest in the region this year. This week, a second national lockdown across England is set to end as new Covid-19 cases fall across most of the country and related deaths start to level off. England will then enter into a phase of regional restrictions, with curbs on hospitality businesses across much of the country.

Mr. Johnson’s battle against Covid-19 has also been personal. In April, he was  hospitalized in an intensive-care unit  and received oxygen after testing positive and showing symptoms of the virus.