China's zero-tolerance approach to Covid-19 is once again being tested as an outbreak of the Delta variant spreads in Fujian province, raising questions about the country's purportedly watertight border and quarantine checks.
The latest outbreak, which was first detected last Friday in the city of Putian, shows signs that it is spreading rapidly.
Cases have more than doubled daily, from 22 on Sunday to 59 the next day, as reported by the National Health Commission.
As at 8am yesterday, there were 139 confirmed cases in three Fujian cities - Putian, Xiamen and Quanzhou. Preliminary tests indicate that the highly infectious Delta strain is behind the current outbreak.
The Chinese authorities have turned to their well-worn playbook of lockdowns, contact tracing and mass testing to curb the spread of the virus.
Xiamen and Putian, cities of five million and 3.2 million residents respectively, have begun mass testing. Schools have reverted to online classes, and residents have been told to stay home.
The spike comes just weeks after China curbed an earlier outbreak of the Delta variant that began at Nanjing airport - and ahead of the week-long National Day holidays which customarily see travellers criss-crossing the country.
The current outbreak was first detected when community tests at a primary school in Putian turned up several positive cases among its pupils - the authorities say many of the children were asymptomatic.
The authorities said the source of the outbreak was the father of one of the pupils, who had returned from Singapore on Aug 4.
The man had completed the full quarantine regimen followed by health monitoring at home.
State media reported that the man tested negative nine times before testing positive last Friday, 38 days after he entered the country.
This led to talk about extra long incubation periods and whether quarantine periods should be extended.
But University of Hong Kong virologist Jin Dongyan said such a long incubation period is unlikely, given that Delta cases typically exhibit shorter incubation times.
It was more likely that the man had become infected while in quarantine, said Professor Jin, noting that the man's viral load was very high when he tested positive, indicating a recent infection.
"While the border control measures are very stringent, there could be accidents - most probably within the quarantine hotels," he said, adding that checks should be done to see if other travellers have been infected.
With highly infectious and transmissible variants like Delta putting increasing pressure on China's zero-tolerance approach, "the lesson is that it is going to be increasingly difficult to have zero cases, to have this zero-tolerance approach", Prof Jin said.
Correction note: An earlier version of this article referred to Xiamen as the provincial capital of Fujian. This is incorrect. We are sorry for the error.