British tourists scrambled to leave Portugal over the weekend in order to beat a Tuesday deadline for a new quarantine imposed by the British government on those returning from Portugal over concerns about a dangerous virus variant.
Britain had recently put Portugal, one of the most popular destinations for British tourists, and 12 other countries and territories with low coronavirus caseloads on a “green list,” allowing visitors coming from Britain to avoid a quarantine period upon returning from those locations.
Britons fatigued by a miserable winter and a four-month national lockdown had just begun flocking to Portugal, because most of the other green-listed places were either not accepting tourists or were not already favored destinations. The process still involved several forms and P.C.R. virus tests, whose costs can total hundreds of dollars.
The decision Thursday to reintroduce restrictions was heavily criticized by British travel operators and opposition politicians. But the government defended the move as a health-safety requirement to help Britain fight a new coronavirus variant that was first detected in India, known now as the Delta variant.
Britain’s switch of travel rules for Portugal prompted thousands of British tourists to pay extra to rebook early return flights. British Airways and other airlines added flight capacity to help bring them home.
As British tourists headed early for the airport in Faro on Sunday, a major tourism hub in Portugal’s southern Algarve region, the line there stretched well outside the terminal, according to reports from British newspapers.
The latest quarantine decision came less than a week after thousands of English soccer fans had visited Porto, in northern Portugal, to watch the final of the Champions League, with no quarantine restriction.
The move by British officials comes as cases remain generally low in Britain, though officials have been working to contain surges of the Delta variant. Daily cases have increased by 89 percent from the average two weeks ago, while deaths have increased by 49 percent, according to a New York Times database.
Dr. Charles Chiu, a virologist at the University of California, San Francisco, said that the rapid rise in Britain of the variant now known as Delta, which was first identified in India, demonstrates just how unpredictable the virus has been. “I don’t envy the public health officials who have to make these decisions, because I really think no one knows much,” he said.
Dr. Chiu, who was the first scientist to discover the variant in California now known as Epsilon, said the key question facing scientists about the Delta variant is how much of a threat it poses to people who are unvaccinated or only partly vaccinated.
This is particularly relevant in Britain, where health officials opted early on to vaccinate as many people as possible by administering only the first dose of a two-dose vaccine. About 60 percent of Britons have received at least one dose, but only 40 percent are fully vaccinated.
“Unfortunately that is the kind of situation where you worry about surges or outbreaks,” Dr. Chiu said.
As the Delta variant spreads across Europe, and as new variants emerge, Dr. Chiu said he expects to continue to see new restrictions and lockdowns as public health authorities respond to emerging threats. “It really points to the importance of getting as many people as possible — globally — vaccinated,” he said.