LONDON — Boris Johnson faces a life-or-death decision. That is not hyperbole. In the next few days, the British prime minister must decide whether to fully reopen society as planned, even as a new and highly infectious coronavirus variant surges.

Johnson will make this decision as British scientists — who are running one of the best genomic surveillance programs in the world — are telling him that the viral strain B. 1.617.2, originally discovered in India and known now as the delta variant, is exploding, and that Britain could soon enter a dreaded third wave.

The delta variant is at least 40% more infectious, Health Secretary Matt Hancock said Monday, and it is quickly becoming the dominant strain in Britain — outpacing the variant first discovered in southeastern England that became ubiquitous in Europe and the United States. Cases of the delta variant are now doubling every eight days.

Is the delta variant more deadly? Does it make people sicker? Unclear.

Will it resist the current arsenal of vaccines deployed in Britain — the AstraZeneca, Pfizer and Moderna shots? Unknown. Early laboratory studies and clinical data suggest it may resist the vaccines a little bit, and people who have been only partially vaccinated may be vulnerable.

But hospitalizations are still low, and ICU beds across the National Health Service system are mostly empty of coronavirus cases. On Monday, England recorded just a single death from the virus.

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That makes it harder to tell the British people they need to wait.

Johnson pledged that it would be “data not dates” that decide when his government should loosen restrictions. Then his government announced a set of target dates, and British people and businesses made plans accordingly.

The last and biggest opening is set to occur on June 21, when “all legal limits on social contact” are lifted, clearing the way for a return to packed theaters, schools, sports arenas, pubs, clubs and trains.

Yet the scientists tracking new cases are advising caution.

“It’s not an easy decision. It’s probably harder than any of the previous ones,” said Paul Hunter, a professor of medicine at the University of East Anglia.

Hunter said he believes Britain is already at the beginning of a third wave. If the delta variant keeps increasing exponentially, “we’re going to get quite high numbers, eventually,” he said.

But Hunter said hospitalization data suggests that there is not a marked increase in acute care for coronavirus patients, “which, to be honest, is probably what many people look at as being the most important criteria.”

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Hunter said he expects Johnson to lift the last restrictions. “And I think that is probably the right decision at the moment. But this time tomorrow, I might change my mind,” he said.

The government is waiting until at least Monday to decide.

Governors in the United States and leaders around the world have had to make these tough decisions about when to lock down and when to open up. But Johnson is especially exposed.

His former top aide, Dominic Cummings, alleged in seven hours of scathing parliamentary testimony that Johnson was “unfit for the job” and that the prime minister’s delay on previous lockdown decisions contributed to “tens of thousands” of avoidable deaths. Overall, more than 128,000 people have died of COVID-19 in Britain.

Cummings described one moment, 10 days before the country went into lockdown, when a powerful official conceded that Johnson’s government had “no plan” and used an expletive to describe the situation.

The government is now running one of the most successful mass vaccination programs in history. That is what the prime minister wants people to remember — not the many U-turns, the botched test-and-trace program, the discarded apps, the mixed messaging or the dearth of protective gear for nurses. And certainly not the explosion of deaths in British nursing homes last year — in part because sick elderly patients were discharged from hospitals without being tested first for the virus.

Delaying the full reopening goes against Johnson’s libertarian instincts. It is sunny now, and the May rains have turned England into a green garden. Johnson does not want to be the prime minister who tells a frustrated population to hold on a few more weeks before enjoying all their freedoms.

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Ahead of Britain’s first lockdown in the spring of 2020, Johnson said it was a “huge wrench” to take away “the ancient, inalienable right of freeborn people of the United Kingdom to go to the pub.”

He reportedly said, when declaring a second national lockdown last October, that he would rather see “bodies pile high” than hit the country with a third lockdown. Johnson denied making the remark.

But even if it was deeply uncomfortable, personally and politically, he has ordered three national lockdowns in the past 15 months. And they have been some of the strictest in the world, at times closing down almost everything, to the point where central London looked like the set for a zombie apocalypse movie.

These lockdowns have been opposed by factions of Johnson’s Conservative Party, who have argued that his mantra “to follow the science” has turned him from brave lion into timid mouse. It is leaders who lead, the critics of lockdown say, and scientific boffins who advise.

Writing in the Times of London, columnist Rachel Sylvester noted that “Johnson’s transformation from libertarian buccaneer to lockdown bureaucrat has infuriated the Tory right-wingers to whom he owes his position as party leader.”

Hancock, the health secretary, said all options remain on the table and that a short delay in lifting all restrictions might be the most prudent course.

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Martin McKee, an expert on public health at the London School of Hygiene and Tropical Medicine, said everything depends on how transmissible the delta variant turns out to be. Whether the virus is 40% more transmissible or 70% more transmissible could have huge consequences.

Even though the young are far less likely to get severe COVID-19, if millions of them are not vaccinated and become infected, hospital admissions could be quite high, McKee said. Some modeling has suggested that a third wave propelled by the delta variant could put more people in the hospital than the first and second waves.

Any decision “will be technical and political, as it should be,” McKee said. He senses that scientific advisers to the government may urge Johnson to lift some but not all restrictions on June 21 — opening up sporting arenas, for example, but insisting that people wear masks on public transportation.

In an interview on the BBC, epidemiologist Neil Ferguson of Imperial College London, whose modeling has guided the Johnson government throughout the pandemic, said the snapshot does not look good but that the data is incomplete.

Asked whether he would fully reopen society, Ferguson demurred. “It’s not my job to make that decision, thankfully,” he said. But he added that the trends last week were moving in “a more negative direction” than the week before. He suggested caution.