Go out, socialize and have fun, South Korea’s government told its people, declaring the start of “a new daily life with Covid-19” — while keeping a vigilant eye out for any sign of backsliding, any need for restrictions to snap back into place.

It didn’t take long.

On Saturday, just the fourth day of the new phase, the mayor of Seoul ordered all the capital’s bars and nightclubs shut down indefinitely after the discovery of a cluster of dozens of coronavirus infections.

South Korea initially attacked the pandemic with such success that it became a model cited worldwide, all but halting a large outbreak without choking off nearly as much of its economy as other nations have. Now it is attempting something just as difficult: moving gradually, safely closer to something resembling everyday life.

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After a 29-year-old man tested positive for the virus on Wednesday, epidemiologists quickly learned that he had visited three nightclubs in Itaewon, a popular nightlife district in Seoul, on May 2. By Saturday evening, they said they were tracking down 7,200 people who had visited five Itaewon nightclubs where the virus might have been spread.

So far, 27 cases have been found among the club-goers and people who had close contact with them, Kwon Jun-wok, a senior disease-control official, said during a news briefing on Saturday.

The mayor, Park Won-soon, cited a higher figure, saying that at least 40 infections had been linked to the nightclubs. As he closed the clubs, he scolded patrons who had failed to practice safeguards like wearing masks, accusing them of putting the entire nation’s health at risk.

“Just because of a few people’s carelessness, all our efforts so far can go to waste,” he said.