Health officials have taken samples from more than 100,000 factory workers during active Covid-19 testing in Samut Sakhon province so far, and about 7,000 of them were positive.
Taweesilp Visanuyothin, spokesman for the Centre for Covid-19 Situation Administration, announced the figures on Wednesday.
Most of the tested workers were at nine big factories, four of which each had 10,000 or more employees. He said 26-27% of workers at some factories were positive for Covid-19.
Officials were also isolating factory workers in good health from the general population to ensure the disease was contained within their workplaces, he said.
They were waiting for these well workers to develop herd immunity, because they might get mild levels of infection from their ill colleagues, Dr Taweesilp said.
Once there was herd immunity, business and other activities could resume in the province, he said.
Samut Sakhon province was the epicentre of the second wave of Covid-19 in Thailand, believed to have begun at the central shrimp market in December.
Since the beginning of the new Covid-19 wave, Samut Sakhon has recorded 15,186 Covid-19 cases, or 80.49% of total cases since mid-December.