These workers likely became a "significant transmission source" for Covid-19 without even knowing it because most in the study were asymptomatic.
The analysis, published Thursday in the journal Occupational and Environmental Medicine, is the first to demonstrate the significant asymptomatic infection rate, exposure risks and psychological distress grocery workers have felt during the pandemic.
In the study, 20% of the 104 grocery workers tested at a store in Boston in May had positive nasal swab tests.
This was a significantly higher rate of infection than what was seen in the surrounding communities, the researchers said. Workers who dealt with customers were five times as likely to test positive for Covid-19 as colleagues in other positions.
But three out of four of those who tested positive had no symptoms.