Philadelphia became the first U.S. city to reinstate its indoor mask mandate on Monday after reporting a sharp increase in coronavirus infections, with the city’s top health official saying she wanted to forestall a potential new wave driven by an omicron subvariant.

Confirmed COVID-19 cases have risen more than 50% in 10 days, the threshold at which the city’s guidelines call for people to wear masks indoors, said Dr. Cheryl Bettigole, the health commissioner. Health officials believe the recent spike is being driven by the BA.2 subvariant of omicron, which has spread rapidly throughout Europe and Asia, and has become dominant in the U.S. in recent weeks.

“If we fail to act now, knowing that every previous wave of infections has been followed by a wave of hospitalizations, and then a wave of deaths, it will be too late for many of our residents,” said Bettigole, noting about 750 Philadelphia residents died in the wintertime omicron outbreak. “This is our chance to get ahead of the pandemic, to put our masks on until we have more information about the severity of this new variant.”

Health inspectors will begin enforcing the mask mandate at city businesses on April 18.

Most states and cities dropped their masking requirements in February and early March following new CDC guidelines. Philadelphia ended its indoor mask mandate March 2.