Testing has uncovered 170 cases of Covid-19 among workers at a pork meat processing plant in Cornwall.
Five hundred staff at Pilgrim’s Pride in Pool, near Camborne, were tested in a contact tracing exercise by the NHS.
Most of those who tested positive were unaware they had Covid-19 and were not displaying symptoms.
They and those they have been in close contact with are isolating in line with government guidelines, said Cornwall Council.
Cornwall remains the area in the region with the highest number of cases.
In the week to Sunday, numbers rose from 115 to 180. The infection rate is 31.5 per 100,000.
Rachel Wigglesworth, interim director for Public Health for Cornwall Council, said: “In finding people who weren’t displaying symptoms we have potentially stopped much wider spread in our communities”.
Council leader Julian German said “proactive testing” was “helping us to take action quickly to limit the spread of Covid in our communities”.
Pilgrim’s Pride said in a statement said “from the outset” it had “worked conscientiously to do all we can to protect our workforce and the local community”.