TORONTO -- Workers at a grocery store in the U.S. tested positive for the novel coronavirus at a rate well above the rest of their community, researchers say.

The research was published Friday in the journal . It is based on a study of 104 adults who were working at a grocery store in the Boston area in May, when they underwent mandatory testing over three days.

Of the 104 workers tracked, 21 tested positive for SARS-CoV-2, the virus that causes COVID-19.

That鈥檚 an infection rate of 20 per cent 鈥 well above the 1.3 per cent detected in the community at the time.

Use of gloves and masks didn鈥檛 seem to make a difference in how likely a worker was to test positive. Neither did social distancing, using public transit or believing COVID-19 to be a greater or lesser threat than others.

What did matter, the researchers found, was what the workers did, and how often their duties brought them into contact with shoppers.

Workers who had 鈥渟ignificant direct customer exposure鈥 鈥 cashiers, cart attendants, supervisors and those working with fresh food 鈥 were found to be five times more likely to test positive than the receivers, cleaners and stockers who rarely if ever interacted with customers.

Only five of the 21 workers who tested positive were displaying symptoms of COVID-19 at the time. With the vast majority of cases both asymptomatic and involving personnel who interact with shoppers, the researchers suggest that this puts both other workers and the public at large at risk.

鈥淥nce essential workers are infected with SARS-CoV-2, they may become a significant transmission source for the community they serve,鈥 they wrote.

This new study, which did not examine how these workers contracted COVID-19 at such a high rate relative to the community, is part of a small but growing body of research looking at COVID-19 spread in individual workplaces.

While the bulk of this research has been conducted in the health-care field, the Massachusetts grocery store is not the first workplace accessible to healthy members of the public to be studied in this way.

A previous study detailed the cases of two hairstylists in Missouri who tested positive for SARS-CoV-2 in May 鈥 around the same time the grocery store workers were being examined 鈥 and seemingly did not pass the virus on to any of the 139 clients they saw before they entered quarantine.

Earlier in the pandemic, a team of researchers in China looked at coronavirus transmission among employees and shoppers at one grocery store there, and found that 9.2 per cent of workers were infected.

The Massachusetts study also looked at workers鈥 mental health, and found that an inability to consistently practise social distancing at work 鈥渨as a significant risk factor for anxiety and depression,鈥 while a commute via public transport was also associated with depression.

The researchers say this backs up calls for supermarket employees and other essential workers to receive help coping with psychological distress brought on by the pandemic.