Philadelphia became the first major 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. 

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