A community of health-conscious, left-leaning Northeasterners, known as a vacation mecca for gay men, Provincetown had one of the highest vaccination rates in the country, upward of 95 percent among permanent residents, Mr. Morse estimates.

On the weekend of July 4, it was also crowded. Around 60,000 people had jammed into a narrow spit of land, where many congregated, maskless, on sweaty dance floors and at house parties…

“We are winding the clock back to maybe April or May of 2021,” said Susan Peskin, a longtime summer visitor who moved there full time four years ago. “Now it is clear, as clear as day, that you can be vaccinated and still get Covid. Bottom line, we have to really watch ourselves and not think it is over. It is nowhere near over.”

By the end of the week, Mr. Katsurinis was taking reports of positive coronavirus cases — all gay men, with an average age of 30 to 35, many of whom who had seen a doctor for other reasons, like flu symptoms or sexually transmitted infections, not suspecting the coronavirus. What puzzled him, he said, was that so many of the infected people were vaccinated.

“I couldn’t believe, frankly, that vaccinated people were getting and spreading it, the way that the contact tracing people were saying,” he said. “I had that moment of saying, ‘I don’t believe that data is accurate.’”