A rmed with her vaccine passport and a giddy urge to celebrate the holiday season, Laura Delgado — and 60,000 other people in Puerto Rico — attended a Bad Bunny concert three weeks ago. Three days later, she was sick with Covid-19, one of about 2,000 people who fell ill as a result of the two-day event. “We did so well; we followed the rules,” said Ms. Delgado, a 53-year-old interior designer. “We followed the mask mandate. Our vaccination rate was so high that we let our guard down. The second Christmas came, we were like, ‘We’re going to party!’”...