China’s National Health Commission reported 61 domestic symptomatic COVID-19 infections on Monday. Many of the cases are believed to stem from one incident in Nanjing where airport cleaning staff were infected sometime around July 10, going undetected for over a week.

At least 15 provinces are reporting new COVID-19 cases; 12 of them have transmissions believed to be linked to the Nanjing airport.

The outbreak represents a significant test as to whether China’s strict public health measures and domestically produced vaccines can beat back a Delta variant that is thwarting containment measures across the region.