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.

Background

The present outbreak is concentrated in Jiangsu province, where 45 of the 61 domestic cases reported on August 2 originated. The discovery of the outbreak ushered in three rounds of mass testing on some 9.2 million people living in the city. Targeted lockdowns have been imposed on districts where transmission has taken place.

Zhangjiajie, a populist tourist destination in Hunan, has emerged as a secondary epicenter, reporting eight cases believed to be linked to Nanjing. The Zhangjiajie cluster is of particular concern to public health authorities because some of the patients were among large indoor audiences, and there’s elevated risk of tourists spreading the virus further afield upon returning home. As of Tuesday, residents and tourists in Zhangjiajie have been barred from leaving the city.

Cases have also been recorded in other popular destination spots, including Hainan Island, Ningxia, and Shandong provinces.