Each employee was tested for SARS-CoV-2, the virus responsible for COVID-19 infection, in May this year as part of a mandatory testing policy across Boston.
They were asked to provide information on COVID-19, including any symptoms and exposure to anyone with confirmed SARS-CoV-2 over the past 14 days.
Information on mental health was gleaned from two validated questionnaires for depression and anxiety.
One in five (21 out of 104) workers tested positive for SARS-CoV-2, indicating a prevalence of 20 per cent at that point in time, the researchers said.
This was significantly higher than the prevalence of the infection in the local community at the time: 0.9-1.3 per cent, they said.
Three out of four of those testing positive (76 per cent) had no symptoms. Of those testing positive, most (91 per cent) had a customer facing role compared with 59 per cent of those testing negative.
Workers in customer facing roles were five times more likely to test positive than their colleagues in other types of role, after accounting for potentially influential factors, such as the prevalence of SARS-CoV-2 where they lived.
Those in supervisory roles were six times more likely to do so, according to the researchers.
This is a small observational study of workers in one store in one city at one point in time, which relied on subjective reports, and as such, can't establish cause, the researchers said.
"This is the first study to demonstrate the significant asymptomatic infection rate, exposure risks, and associated psychological distress of grocery retail essential workers during the pandemic," the researchers said.
"Once essential workers are infected with SARS-CoV-2, they may become a significant transmission source for the community they serve," they said.
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