Philadelphia became the first major U.S. city to restore its indoor mask rules on Monday after reporting a sharp rise in coronavirus infections, and the city’s top health official said she wanted to stem a potential new wave driven by omicron sub-variants.

Health Commissioner Dr. Cheryl Bettigole said confirmed COVID-19 cases had increased by more than 50 percent in 10 days, the threshold at which city guidelines require people to wear masks indoors. Health officials believe the recent spike was driven by a highly transmissible BA.2 subvariant of omicron, which has spread rapidly in Europe and Asia, and has dominated the United States in recent weeks.

“If we don’t act now, knowing that every previous wave of infection has been followed by a wave of hospitalizations and then a wave of deaths, it will be too late for many of our residents,” Bettigole said, noting that Philadelphia approx. There are 750 inhabitants. Residents died in the winter omicron outbreak. “This is our chance to get ahead of the pandemic with masks until we have more information about the severity of this new variant.”

Health inspectors will begin enforcing mask-wearing rules at city businesses on April 18.

Most states and cities dropped mask-wearing requirements in February and early March, according to new guidelines from the U.S. Centers for Disease Control and Prevention that focus less on case numbers and more on hospital capacity. The CDC said at the time that as the virus subsides, most Americans can safely remove their masks.

Philadelphia ended its indoor mask mandate on March 2, with Bettigole admitting “it’s great to feel that normal feeling again.”

Confirmed cases have since risen to more than 140 a day — still a fraction of what Philadelphia saw at the peak of the omicron surge — while there are only 46 COVID-19 patients in the hospital. The CDC says community transmission remains low in Philadelphia, a level the agency says is an option to wear masks.

The restaurant industry opposes the city’s reimposed mask rules, saying workers will bear the brunt of customer anger over the new rules.

“This announcement is a major blow to thousands of small businesses and other operators in the city who hope this spring will be the beginning of a recovery,” said Ben Fileccia, senior director of operations for the Pennsylvania Restaurant and Lodging Association.

The Policy Lab at the Children’s Hospital of Philadelphia said Friday that although transmission is expected to increase in the northern U.S. in the coming weeks, admissions remain low, and “given the hospital’s good capacity, our team recommends against wearing masks.”

Requiring people to wear masks will help restaurants and other businesses stay open, Bettigole said, while a new wave of COVID-19 will keep customers at home. She said hospital capacity was only one factor in her decision to reinstate the mandate.

“I sincerely hope we don’t have to do it again,” Bettigole said. “But I am very worried about our vulnerable neighbours and loved ones.”

In New York City, Mayor Eric Adams paused efforts to ease many of the city’s virus regulations as cases rose, and now opts to continue providing masks to children ages 2 to 4 in city schools and preschools. But Democrat Adams said New Yorkers shouldn’t let the pandemic affect their lives, and he has lifted most other mask requirements and rules requiring proof of vaccinations when dining at restaurants, working out at the gym or attending shows .

Adams was asked during a virtual news conference Monday afternoon whether he would consider reimposing New York City’s mask rules based on Philadelphia’s decision. The mayor said he would take advice from his team of doctors on whether to restore any restrictions. Adams himself tested positive for COVID-19 on Sunday.

New York City is now averaging about 1,800 new cases a day, about three times higher than when New York began relaxing its rules in early March. That doesn’t include many home tests that aren’t reported to health officials.

The latest outbreak has hit many high-profile officials in Washington, including Cabinet member and House Speaker Nancy Pelosi, as well as the governors of New Jersey and Connecticut. Some universities have reinstated their mask-wearing mandates.

D.C. health officials said they have no immediate plans to change virus protocols, but they reserve the right to change course in the future.