BEIJING — Authorities in China’s financial hub of Shanghai have quarantined 186 people and conducted virus tests on more than 8,000 after a freight handler at the city’s main international airport tested positive for the coronavirus.

The city government said on its microblog Tuesday that it remains unclear how the 51-year-old man contracted the virus, which has largely spared the sprawling metropolis despite its dense population and strong international links. It says no additional cases have been found.

In China’s northern port city of Tianjin, more than 77,000 people have been tested after a locally transmitted case was reported there Monday. That case was believed to be linked to a cold storage warehouse, reinforcing suspicions that the virus may be spreading to victims from frozen food packaging.