PHILADELPHIA >> A dozen new coronavirus cases in the Philadelphia area have been traced to someone who attended gatherings at beach houses at the Jersey Shore, health officials said.

Eleven cases reported Saturday were linked to a New Jersey resident at gatherings in the past two weeks, Bucks County officials said. One case reported Friday also was traced to the person.

It’s an important reminder not to let one’s guard down at the beach, said Dr. David Damsker, health director of the large county, which borders Philadelphia to the south and New Jersey to the east. He did not disclose exactly where the gatherings took place.

The region’s mass transit system today reinstated a requirement that passengers wear masks. The Southeastern Pennsylvania Transportation Authority had eased the requirement to a recommendation in April after a viral video showed a rider being dragged off a bus by police after boarding without a mask.

Employees will now remind riders of the requirement, SEPTA said.