LONDON — Britain is now, essentially, one big, high-stakes science experiment.

It is putting multiple vaccines to the test amid one of the world’s worst coronavirus outbreaks, propelled by a variant of the virus that is more contagious and possibly more deadly than the original.

Britain is ahead of most countries in the vaccination race, but it is gambling that it can extend the interval between two doses, to stretch limited supplies.

And while those vaccinations are underway, Britain is trying to determine how rigid and long a shutdown must be to inhibit the virus as it has evolved. The variant now dominant here has been found in at least 50 other countries, including the United States.

So, pay attention, world. The findings that emerge from here in the coming weeks and months will have critical implications for you, too.

As of Tuesday, more than 6.8 million people in the United Kingdom had received the first of two doses of a coronavirus vaccine — either from Pfizer-BioNTech or the homegrown Oxford-AstraZeneca jab — and 472,000 others have gotten the second booster shot, according to the government’s daily summary. That’s more inoculations per capita than the United States or any other country in Europe. (The Moderna vaccine is authorized here, too, but won’t arrive until March.)

British Prime Minister Boris Johnson said on Jan. 26 that he was "deeply sorry for every life that has been lost," as the coronavirus death toll passed 100,000. (Reuters)

At the same time, Britain has surpassed 100,000 dead, and on many days, it records the highest per capita death toll from the coronavirus on the planet, as it desperately tries to keep its hospitals from being overwhelmed.

Asked by a reporter what went so wrong, Prime Minister Boris Johnson said at a Tuesday evening news briefing: “I’m deeply sorry for every life that has been lost, and, of course, as prime minister, I take full responsibility for everything the government has done. What I can tell you is that we truly did everything we could and continue to do everything that we can.”

The entire country is now in its third week of its third national shutdown, with no idea when strict stay-at-home orders and school closures will be eased — and by how much.

April may be overly optimistic, scientists warn.

Yet even as it struggles, Britain remains a scientific powerhouse, with some of the best infectious-disease surveillance and modeling in the world, coupled with a cutting-edge consortium tracking the emergence of new variants. It also has a well-run national health-care system, which is collecting data available to researchers.

British scientists reasonably expect to be among the first to answer some of the big outstanding questions of the pandemic: How well do vaccines, shown to be safe and effective in clinical trials, work in the real world? Do they save lives? Lessen disease? Block transmission? Will vaccines alone be enough to end shutdowns?

A real-life test for coronavirus vaccines

Britain’s vaccination campaign began on Dec. 8, and scientists say they should soon be able to measure how well at least the first shots are protecting the population.

Early results from Israel, which is also moving quickly to vaccinate, suggests that a first dose of the Pfizer-BioNTech vaccine reduced positive tests for the coronavirus by a third.

The vaccination rollout in Britain has gone surprisingly smoothly, given the government’s failure to launch a “world-beating” test-and-trace program and its stumbles in getting personal protective equipment into hospitals in the early days of the pandemic.

Britain’s vaccinations are now taking place at more than 1,300 sites, including hospitals, doctor’s offices and pharmacies, horse-racing courses, town halls, cathedrals, arenas, a shuttered cinema and a mosque.

Experts say that although there have been some problems — London Mayor Sadiq Khan complained that the hard-hit capital hasn’t received its fair share of doses yet — the country has been able to scale up quickly, in part because of its centralized, top-down National Health Service system.

“If you tell 30,000 GP doctors to do something, they will do it, and you can make it happen relatively quickly,” said Nigel Edwards, chief executive of the Nuffield Trust, a London-based health think tank.

Officials aim by mid-February to have given a first vaccine dose to 15 million people, including nursing home residents and caregivers, front-line health-care workers, those with other conditions that put them at extreme risk, and everyone age 70 and older.

But to get vaccines to as many people as possible, the government is delaying second doses — administering boosters up to 12 weeks out, rather than the recommended three to four weeks.

AstraZeneca’s trials included some doses scheduled as far as 12 weeks apart. Pfizer, though, has warned that it doesn’t have evidence to support such a long delay, and the British Medical Association on Sunday said the second doses of the Pfizer vaccine should be delayed no more than six weeks.

Other countries are watching the British experiment, hoping success here will mean that they, too, can spread available doses further while waiting for additional shipments.

Studying a more contagious strain

Pfizer and Moderna have said laboratory testing showed that their vaccines would be effective against the virus variant that has emerged in Britain. But researchers caution that the hopeful results must be seen in the real world.

The variant, first detected in London and southeast England, may be 30 to 70 percent more transmissible, having evolved to bind more tightly to human cells, easing entry and replication, virologists have found. It may also be more deadly.

“It’s really a serious turn for the worse, unfortunately,” said John Edmunds, a professor of disease modeling at the London School of Hygiene and Tropical Medicine.

Patrick Vallance, England’s chief scientific adviser, said that among 1,000 men age 60 or older who get infected, the original virus would kill 10. The new variant, he said, would kill 13 or 14 — an increase of at least 30 percent.

Understanding of the variant is still in its early stages. Preliminary research suggesting that it might spread more effectively in children than the original virus is being examined. But the early data, examining the fates of 50,000 patients here, appear robust, said Edmunds, who led one of the studies that prompted a government announcement about higher mortality on Friday.

Because the variant appears to spread more easily through the population, it leads to more infections, which send more people to the hospital, where more of them might die.

But Edmunds said it appears as though the variant itself is more lethal. He and other disease modelers said they were careful to screen out other possible causes for higher mortality, such as how busy a hospital might be.

The scientists said they must await larger data sets to see if younger people also face higher mortality.

How long will shutdowns be needed?

The government ordered a third national shutdown on Jan. 4, after it was clear that regional restrictions weren’t sufficient to contain the spread of the variant.

People are supposed to stay in their homes except in special circumstances. Mixing between households is banned. Schools are closed, as are all but essential shops.

Three weeks in, the shutdown has begun to reduce the increase in new infections, even as the numbers of hospitalizations and deaths continue to soar.

There is growing pressure on the prime minister to lift restrictions as soon as possible. But British health experts warn that getting through this wave of infections, especially against new variants, may be much harder, and take much more time, than previous spikes.

They advise that even with a mass vaccine rollout, mask use, social distancing and other restrictions should continue through spring and probably into the summer. They caution that some measures may need to be kept in place until the fall, when Britain promises to have all adults vaccinated.

Matt Keeling, a modeler at the University of Warwick and member of the government’s science advisory team, said 100 times as much virus is circulating now as in the summer, making more people more vulnerable, despite the deployment of vaccines.

“Vaccines are not a panacea,” said Keeling, who added that scientists still do not know how protective the shots will be after the first and second doses and whether the vaccines can stop the chains of transmission.

Mark Woolhouse, a professor of infectious-disease epidemiology at University of Edinburgh who also advises the British government, said a quick release from shutdown in April could lead to “a huge wave of infections.”

For example, Woolhouse said that if even 90 percent of Britain’s 10 million most vulnerable people receive an effective vaccine, that would still leave 1 million without any immunity.

Researchers assume that as more and more people are protected against the virus by immune responses generated by vaccines or past infections, herd immunity will begin to offer some respite, by blocking the virus’s ability to move from person to person freely.

When that threshold will be reached for the coronavirus is unknown.