Armed 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 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!’ ”

The superspreader concert helped usher in an explosion of COVID-19 cases in Puerto Rico, which until then had been celebrating one of the most successful vaccination campaigns in the United States. The concert was one of a series of business events, company holiday parties and family gatherings that fueled a 4,600% increase in cases on the island, a surge that public health officials worry could linger into the new year; the Puerto Rican holiday season stretches to Three Kings Day on Thursday.

While the omicron variant has besieged the entire country, it is especially worrisome in Puerto Rico, a U.S. territory already overwhelmed by government bankruptcy, an exodus of health professionals and a fragile health care system. Officials imposed a new wave of tough restrictions on travelers and diners in hopes of staving off the new wave of cases.

Rafael Irizarry, a Harvard University statistician who keeps a dashboard of Puerto Rico COVID-19 data, posted the daunting facts on Twitter: One-third of all coronavirus cases the island has recorded since the start of the pandemic occurred in the past month. The number of cases per 100,000 residents jumped to 225, from three, in three weeks.

In December, the number of hospitalizations doubled — twice.

Without the polarizing politics that have plagued the debate over vaccines in other parts of the country, nearly 85% of those in Puerto Rico have received at least one vaccine dose, and about 75% have gotten both shots.

But in the face of a highly contagious new variant, a high vaccination rate is not that meaningful anymore, Irizarry said. Most in Puerto Rico have passed the six-month limit beyond which the vaccine’s effectiveness begins to wane, yet at least 40% have yet to receive their booster shots, health officials said.

At one point last week, the daily case count had surpassed 11,000, a very high figure for an island with just 3.2 million inhabitants. The exponential increases have begun to taper off, but case numbers are still climbing, Irizarry said.

“I first noticed something going on on Dec. 13, and I alerted the Department of Health,” he said. “By the 14th and 15th, it was obvious. I called the guy who runs the database and said, ‘Is there some kind of glitch in the database?’ ”

There are currently 317 people hospitalized with COVID-19, more than a quarter of whom are children, according to the island’s Department of Health. That’s about half the number of people who were hospitalized with the illness at this time last year before so many people were vaccinated. But it is still proving to be a challenge for hospitals.

“The problem is, let’s suppose omicron is half as bad,” Irizarry said. “If you have eight times more cases, the math doesn’t work out in your favor.”

Gov. Pedro R. Pierluisi has ordered lower capacity limits in restaurants. To attend large public events, people now have to be vaccinated and present a negative COVID-19 test. Passengers arriving on domestic flights must show a negative test taken within 48 hours before arrival, regardless of their vaccination status. Similar rules were already in place for international flights.

Mass public events, including an important celebration to commemorate the 500th anniversary of the island’s capital, San Juan, have been canceled. “Dick Clark’s New Year’s Rockin’ Eve,” which ABC had planned to broadcast live from Puerto Rico in front of big crowds, was downgraded to a virtual event.

After a few dozen Miss World contestants got sick, the pageant finals in Puerto Rico were canceled.

On Thursday, the Scientific Coalition, a group of scientists and health professionals that has been advising the governor, recommended even stricter measures, such as limits on alcohol sales and shorter hours for bars and other establishments. On Friday, the governor followed the recommendation and ordered businesses closed between midnight and 5 a.m. from Tuesday until Jan. 18. He also mandated booster shots for restaurant employees and public safety workers.

“It’s a message that’s hard to digest when two weeks ago the case numbers here were among the lowest in the world,” said Daniel Colón-Ramos, a Yale University professor who is president of the coalition. The measures are particularly hard in Puerto Rico, he said, where it is hard to overstate the importance of a holiday season that starts at Thanksgiving and lasts until Jan. 6. He described it as “Fourth of July plus the Super Bowl.”

“Christmas is a week that Puerto Ricans celebrate their identity,” he said. “They celebrate their family. They celebrate their faith. They celebrate their heritage.”

The average age of people who become infected on the island is 33, health officials said. But experts worry that if young people who become infected while attending parties and other events visit elders for New Year’s and Three Kings Day, the number of sick older people is certain to rise. With so many of its young professionals moving in recent years to Florida, Texas and other states, Puerto Rico has a disproportionately high percentage of older adults, many of whom suffer from diabetes, obesity and other ailments that put them at higher risk for coronavirus complications.

“We have a health system that is — it’s not a secret — fragile,” said Carlos R. Mellado López, the island’s secretary of health. He urged people not to unnecessarily overwhelm testing centers and insisted that Puerto Rico had the tools necessary, such as monoclonal antibody treatments, to combat the crisis.

But experts also caution that thousands of medical professionals have left Puerto Rico in recent years in search of higher salaries, which could complicate the island’s ability to attend to large numbers of sick people. The number of doctors on the island has dropped by 5,000 since 2006, and another exodus of primary care doctors is anticipated because they were left out of recent tax incentives designed to keep specialists from leaving, said Víctor M. Ramos Otero, president of the Puerto Rico doctors’ association.

“The problem we have is not the beds,” Ramos said. “The principal issue is the personnel.” José R. López de Victoria, a public health researcher who helped design coronavirus protections for Puerto Rican basketball teams, said the crisis was still stretching ahead.