Britain became the first Western country to begin a mass effort to inoculate people against the coronavirus on Tuesday morning, less than a week after granting emergency authorization for use of a vaccine made by U.S. pharmaceutical giant Pfizer and German biotech firm BioNTech.

“It’s the best early birthday present I could wish for, because it means I can finally look forward to spending time with my family and friends,” said Margaret Keenan, 90, who was the first person to receive the vaccine outside of clinical trials.

China and Russia have been marketing vaccines of their own around the world, but they have not been subject to the rigorous standard of testing that many governments require.

Why did Britain beat the rest of the West?

British drug regulators are swift by reputation. They rely more heavily on data summaries submitted by companies than do some of their counterparts, according to experts, and have more flexibility to review data as it is released and conduct multiple reviews simultaneously.

Regulators were able “to look at that data as it’s come through and do things in parallel,” British Health Secretary Matt Hancock said in an interview with Times Radio last week.

He also claimed that Britain’s exit from the European Union allowed a faster approval, a notion that experts debunked. Other E.U. countries could issue their own emergency authorizations but have declined to do so and have instead chosen to follow the E.U.’s collective process. E.U. leaders feared unilateral actions would undermine trust in the vaccine.

The United Kingdom began a rolling review of vaccine data in October, but so did the E.U. Experts suspect that the main reasons for the differences in timing are procedural hurdles.

E.U. drug regulators must assess by committee the independent views of 27 member states, Penny Ward, a visiting pharmaceutical researcher at King’s College London, told the Science Media Center.

How do the U.K. and U.S. approaches differ?

The U.S. Food and Drug Administration’s vaccine authorization procedures are slightly more complex than Britain’s.

“The FDA rule requires the agency to complete an internal assessment and to consult an external advisory board prior to making a decision,” said Ward.

“Although not strictly required for emergency use authorization, because of the public health implications of potential very widespread use of a vaccine, they have already stated that this will form part of their actions,” she said.

The two countries will probably diverge in who gets vaccinated first. While the United States is likely to include health-care workers in the highest-priority group of people to be vaccinated, the U.K. has opted not to include most of them, in favor of nursing home caregivers and people older than 80, who are at a high risk of dying from covid-19 — a decision that prompted frustration among some nurses and doctors last week.

U.K. officials acknowledged last week that vaccinations inside nursing homes, also a high priority, will be a logistical challenge. At first, hospitals are expected to function as vaccination hubs, so some nursing home residents may have to wait.

Why was the British government so eager to authorize first?

After a muddled pandemic response, Britain welcomed the opportunity to recast the narrative.

In a tweet last week, British Business Secretary Alok Sharma wrote that the approval meant the U.K. had “led humanity’s charge against” covid-19. Prime Minister Boris Johnson also struck a celebratory tone, saying that the country now knows “that we will succeed and together reclaim our lives and all the things about our lives that we love.”

Launching the West’s first vaccination program could help Johnson secure a much-needed political victory after a year in which Britain recorded one of the highest coronavirus death tolls worldwide.

How has the rest of Europe responded?

Britain’s vaccine rollout has prompted skepticism in much of the rest of Europe, where it stoked concerns that other E.U. member states could be tempted to preempt the E.U. regulator’s timeline. Such unilateral action, some fear, would boost vaccine skepticism.

Continental Europe faces more of an uphill battle in convincing people to take the vaccine than Britain does.

Around one-fifth of U.K. respondents in a large-scale study, conducted between March and September, said they likely won’t get vaccinated against the coronavirus. But in France, at least half of all respondents in several recent surveys said they had no intention to get vaccinated against the coronavirus. In Germany, 29 percent said they likely or certainly won’t get a vaccine.

Scientists say around two-thirds of the population need to be vaccinated to reach herd immunity against the virus.

How far ahead will early authorization put Britain?

Britain’s early vaccine rollout is likely to provide lessons for other countries, but it’s unclear how much of an impact the timeline will make in the medium- or long-term.

In the U.K., a country of more than 66 million, authorities expected 800,000 Pfizer-BioNTech vaccine doses to be available by Tuesday — sufficient for around 400,000 people because it requires an initial dose followed by a booster three weeks later.

The initial wave of inoculations could provide protection against a winter spike in cases among the most vulnerable. Over time, however, who authorized when could become something of a moot point as logical challenges and material disparities come to dominate the question of which countries will be able to vaccinate large parts of their populations more swiftly than others. The Pfizer-BioNTech vaccine needs to be stored at minus-75 degrees Celsius (minus-103 Fahrenheit), making its distribution challenging for many countries.

Starting next year, British authorities are expected to open immunization centers in sports arenas and other locations, where the majority of vaccines may be injected.

The U.K. isn’t the only country already preparing for mass immunization. In Germany, for instance, authorities began constructing purpose-built mass vaccination centers weeks ago. And in the United States, the federal government has said it could distribute over 6 million Pfizer doses within 24 hours after an FDA emergency-use authorization is issued.

Who is next?

The second country to approve the Pfizer vaccine for emergency use last week was Bahrain, even though it remains unclear when and how the small Persian Gulf nation will be able to launch its vaccination campaign.

Other countries, including China and Russia, are moving ahead with their own vaccines. Moscow began inoculating some high-risk workers over the weekend — despite lingering questions over the safety of its vaccine.

In the United States and the E.U., regulators are expected to make a decision on the Pfizer-BioNTech vaccine in the coming days or weeks. Emergency authorizations could also be granted for the respective vaccines by U.S. drugmaker Moderna and by Swedish-British company AstraZeneca.

AstraZeneca’s Indian production partner and Pfizer have applied for emergency authorization in India, where officials said Monday that an accelerated review was underway. The AstraZeneca vaccine does not need to be stored at extremely cold temperatures, which would give it an advantage over the Pfizer doses in India and many low-income countries.