BEIJING - 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 (Sept 10) 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 on Tuesday, the provincial authorities said 139 confirmed cases have been reported in three Fujian cities, which, besides Putian, include 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.
Students have been told to stay home as schools revert to online classes, while movement into and out of residential areas is restricted.
The current spike in cases comes just weeks after China successfully curbed another outbreak of the Delta variant that began at Nanjing airport - and as the country gears up for the week-long National Day holidays which customarily see travellers criss-cross the country.
The National Health Commission told state broadcaster CCTV that the current situation was "severe and complex", with risks of the virus spreading to other areas.
It pointed out that some 30,000 people have left Putian for other provinces between Aug 26 and Sept 10.
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 a 14-day quarantine in Xiamen, another seven days in Xianyou county in Putian, and a further seven days of health monitoring at home.
State media reported that he tested negative nine times, before he tested positive last Friday, 38 days after he entered the country.
It has led to talk in China about extra long incubation periods and whether quarantine periods should be extended.
But experts such as University of Hong Kong virologist Jin Dongyan said such a long incubation period is unlikely, given that Delta cases typically exhibit shorter incubation times.
Instead, 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 he was recently infected.
"While the border control measures are very stringent, there could be accidents - most probably within the quarantine hotels," he said.
Environmental samples of quarantine hotels should be done, and checks done to see if other travellers have been infected, Prof Jin added.
That periodic community testing had picked up the cases in the Putian primary school showed that these tests were "complementing" existing quarantine regimes, and these tests should be expanded to other community groups, he added.
But concerns remain that the virus could spread widely among children, who are largely unvaccinated.
The outbreak has shown how highly infectious and transmissible variants like Delta are putting increasing pressure on China's continued zero-tolerance approach.
"The lesson is that it is going to be increasingly difficult to have zero cases, to have this zero-tolerance approach," he 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.