Health officials in four provinces have been ordered to monitor people linked with the new coronavirus hotspot at Bang Khae Market.
Disease Control Department director-general Opas Karnkawinpong said on Sunday health personnel in Suphan Buri, Nakhon Pathom, Phetchaburi and Samut Sakhon were assigned to closely follow up the cases after the department had sent the names and personal details to them.
Mass testing at the market has found 85 infections so far, including five people from Suphan Buri, three from Nakhon Pathom, six from Phetchaburi and one from Samut Sakhon, department figures showed.
The patients comprised 46 Thai nationals, most of them vendors, and 39 Myanmar citizens working at the market. City Hall's Facebook page said the results of some 3,000 tests are pending.
"Most of the infections were asymptomatic," Dr Opas said.
The discovery at Bang Khae Market was the latest cluster to put health authorities on alert.
Bang Khae Market in Bang Khae district comprises six small markets, including one named Wonder, built in the same compound. Wonder Market, the source of the new surge, was ordered to close but other markets in the area are still open.
Bang Khae is the third market to become a Covid-19 hotspot following the outbreaks at Pornpat in Pathum Thani last month and Samut Sakhon in December. The outbreak at Pornpat market originated in Samut Sakhon but the source of the infections at Bang Khae remains unclear.
The country added 170 new cases on Sunday, more than half of them from Bang Khae Market.