The contagious virus has already reached elsewhere in China and abroad, and even an authoritarian government has only a small timeframe in which trapped residents will submit to such a lockdown, they say.

“I think we have passed the golden period of control and prevention,” said Guan Yi, an expert on viruses at Hong Kong University…

Yi, who returned from Wuhan just before the lockdown, pointed out huge numbers of people would have already left ahead of the Lunar New Year holiday, which began Friday.

They could have been incubating the virus “on their way out of Wuhan”, he said.

Symptoms could take several days to emerge — effectively seeding a health time-bomb across the country and abroad.