The split is particularly stark in Britain, which is facing the spread of a new variant, while America has essentially lifted all rules for people who are vaccinated.
LONDON — Over Memorial Day weekend, 135,000 people jammed the oval at the Indianapolis 500. Restaurants across the United States were thronged with customers as mask mandates were being discarded.
The formula, which gained the Biden administration’s blessing, was succinct: In essence, if you are fully vaccinated, you can do as you please.
But while the United States appears to be trying to close the curtain on the pandemic, across the ocean, in Britain and the European Union, it is quite a different story.
Despite plunging infection levels and a surging vaccine program, parts of Europe are maintaining limits on gatherings, reimposing curbs on travel and weighing local lockdowns.
In Britain, the spread of a new, highly contagious variant first detected in India has scrambled calculations just as the country planned to return to something more like prepandemic life on June 21.
Parts of Britain have decided to extend lockdown restrictions. This past week, the government tightened its travel rules, including for the fully vaccinated, by removing Portugal — the most popular remaining European tourist destination — from the list of places where Britons could fly without stringent quarantines.
And scientists are heatedly debating whether to go ahead with a June 21 reopening, with some saying that the costs of delaying it by a few weeks would pale in comparison to the damage that could be wrought by giving the variant first detected in India, known as Delta, extra opportunities to spread while people are still acquiring immunity.
Though vaccinations got off to a slow start in much of Europe, they have since helped drive down cases, as in the United States. Nevertheless, on the fundamental question of how to approach an end to coronavirus restrictions, America and Europe have diverged.
“We’re now looking at a variant where we have less knowledge about its properties,” Theo Sanderson, a researcher at the Wellcome Sanger Institute, said of Delta. “It just means we have less certainty about what things will look like going forward.”
Britain has become the world’s most sophisticated laboratory for the virus’s evolution, with 60 percent of England’s coronavirus cases being analyzed through genomic sequencing. That has allowed the country to pick up on the earliest signs of dangerous variants, and made Britain a harbinger of the challenges facing even heavily vaccinated nations as newer versions of the virus reach the unvaccinated.
While scientists are at odds over exactly how serious a threat the Delta variant poses to Britain, fears over its potential to undo some of the country’s hard-won progress toward reopening have crescendoed.
“The British are worrying more than any other country,” said Tim Spector, a professor of genetic epidemiology at King’s College London. “We seem to be much more receptive to the doomsday scenarios than they are in the U.S.”
Since the Delta variant arrived in Britain in March, it has rapidly outspread other versions of the virus, including the very contagious variant first identified in Britain that contributed to deadly waves around the world this winter. That, in turn, has created localized outbreaks that have nudged Covid cases up.
A top scientific adviser to the British government estimated on Friday that the Delta variant was roughly 60 percent more contagious than the earlier one from Britain. Health officials also warned that cases caused by the Delta variant might lead to a higher risk of hospitalization, though it was too early to say for certain.
The divergent strategies of European nations and the United States also reflect broader differences in how Western governments are thinking about their responsibility to unvaccinated people, scientists said.
Many American states began dramatically scaling back restrictions not long after they made all adults eligible for vaccines — whether or not levels of uptake were as high as desired. The economy has reopened, and with fully vaccinated people more protected against the Delta variant, the United States seems to be in a strong position to limit its spread.
Scientists, though, worry that the variant could soon gain ground in unvaccinated pockets of the United States, where the virus continues to sicken and kill people at elevated rates. The Biden administration is still searching for ways to overcome that vaccine hesitancy.
In Britain, even with more than 90 percent of people over 65 having been fully vaccinated, health officials have resisted as speedy a reopening as they seek to expand inoculation rates in lower-income and nonwhite areas.
“We know the virus predominantly hits poorer communities and people of color hardest,” said James Naismith, a structural biologist and the director of Britain’s Rosalind Franklin Institute, a medical research center. “The U.S. strategy perhaps reflects a more deep-rooted commitment to individualism. The U.K.’s vaccination campaign is highly managed and mirrors more a sense of being our brother’s keeper.”
Britain decided last year to delay second vaccine doses to give more people the partial protection of a single dose. That helped it weather the wintertime surge but also left it potentially exposed to the Delta variant. Health officials said this past week that there was strong evidence of “a reduction in vaccine effectiveness” for the new variant that was most pronounced after a single dose.
Health officials have since changed the guidance to speed up second doses, but many scientists are urging the government not to commit to reopening until the impact of the variant becomes clearer.
While indoor dining has resumed, most groups larger than six are prohibited, and nightclubs, music venues and large events remain shuttered, leaving many hospitality businesses still reeling. The British government has long targeted June 21 — “freedom day,” in the parlance of the tabloids — as the date when it hoped “to remove all legal limits on social contact.”
The critical issue is whether a recent rise in Covid cases will accelerate and translate into a surge of severe illness, something that depends in part on how quickly people are given their second doses. Scientists reported this past week that Pfizer’s vaccine elicited a weaker antibody response to the Delta variant than the original virus, especially among older people, raising the prospect of a booster shot being needed.
“There are reasons to be hopeful — we’re not seeing a big trend in hospital admissions — but it’s early days,” Professor Naismith said. “If we don’t see anything by June 14, we can exhale. We don’t need to hold our breath.”
Other scientists argue that widespread vaccinations have changed the reopening calculus. While only half of British adults are fully vaccinated, that includes nearly all of the most vulnerable. And 76 percent overall have gotten one shot. As a result, some scientists say, upticks in new infections are tolerable so long as the vast majority do not lead to serious illness or death.
“This variant is going to find it hard to spread, because it’s limited to younger people and limited to certain parts of the country,” Professor Spector said.
He said the government needed to help the neighborhoods where it was spreading and, beyond that, encourage people to keep working from home and socially distancing when possible. But delaying the easing of restrictions, he said, was not necessary.
“We need to get used to the idea there will be a few thousand cases every day and that this is a part of our life,” Professor Spector said. “Those cases will be milder.”
While many Britons grudgingly accepted the need for lockdowns when hospitals were overwhelmed, Prime Minister Boris Johnson’s government has felt mounting pressure to stick to the June 21 reopening. Even its decision to rein in trips to Portugal caused outrage: “Brits’ Foreign Holidays Nightmare,” one tabloid front page screamed on Friday.
In the European Union, where vaccination levels still lag behind those in America and Britain, officials are also being cautious. Germany, France and Austria all moved quickly to bar most visitors from Britain.
Like Britain, the bloc was chastened by a surge of the variant from Britain this winter that contributed to one of the world’s highest death tolls. Governments were hammered for failing to cement the gains of last summer, when lockdowns were lifted across most of Europe.
In the bloc, 47 percent of the adult population has received a first dose, according to the European Center for Disease Prevention and Control, but only 23 percent have full protection.
For those reasons, European leaders have said that vigilance is needed, even though infections have fallen about 80 percent since mid-April.
“This progress is fragile,” Hans Kluge, the World Health Organization’s director in Europe, warned last month. “We have been here before. Let us not make the same mistakes that were made this time last year.”
Still, now that supply bottlenecks have eased, European officials are confident that 70 percent of adults will be fully vaccinated by July.
The quandary that Europe faces over how to react to the Delta variant may recur as the virus continues to evolve, some scientists said. As long as it remains in wide circulation, even more transmissible variants could emerge, forcing countries to grapple with whether to hunker down yet again or risk the virus spreading through unprotected populations.
Poorer nations are facing far more difficult choices, though. If the same sort of lockdowns that controlled the variant from Britain prove insufficient against this new one, those countries could have to choose between even more draconian and economically damaging shutdowns or even more devastating outbreaks. The Delta variant has already taken a horrifying toll on South Asia.
“Globally, it’s a nightmare, because most of the world is still not vaccinated,” said Jeremy Kamil, a virologist at Louisiana State University Health Shreveport. “It raises the stakes.”