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.
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But three out of four of those who tested positive had no symptoms.
"We were definitely surprised to see that there were that many people that were asymptomatic," said Dr. Justin Yang, an assistant professor at Boston University School of Medicine and a researcher at Harvard School of Public Health who worked on the study. "This is definitely very alarming as it means that retail grocery store employees are exposed to customers and sort of serve as a middleman for the virus - like a super spreader almost."
Workers in the study had tried to take precautions. Nearly all, 91%, said they wore a face mask at work and 77% said they also wore masks outside of work. Yet only about 66% said they were able to practice social distancing consistently on the job.
This inability to social distance had an emotional, as well as a physical impact. Nearly a quarter of the people in customer service jobs said they had problems with anxiety and depression compared to 8% of workers who did not have to interact with customers. Employees who commuted to work by bike, car or by walking were less likely to experience depression than those who used public transportation, the study found.
"If you are in an environment when you're literally in front of a customer, you can't be more than six feet and that is really stressful for essential employees," Yang said.
At least 108 grocery workers have died and more than 16,300 have been infected or exposed to Covid-19, the United Food and Commercial Workers International Union, or UFCW, said Thursday. The union represents 1.3 million employees.
The rates of infection among the workers in this study do seem high, Yang said. By comparison, an earlier study of Covid-19 infections among Dutch health care workers found the infection rate was about 10%.
Yang said he hopes this study prompts the government and store owners to provide better guidance, routine testing and protection for grocery store workers.
There has been a national movement to designate grocery workers as first responders which would give them priority access to testing and personal protective equipment.
In an editorial for CNN in August, Marc Perrone, the President of UFCW and Democratic Vice Presidential candidate Kamala Harris argued that grocery workers should also get hazard pay.
Non-union grocery workers often have little to no healthcare coverage, meaning they could potentially face expensive health care bills if they contracted COVID-19.
Some states have increased support for grocery workers by increasing access to childcare and requiring shoppers to wear masks. Three states offer free testing for these workers and four offer worker's compensation, according to UFCW, but none of the states provide the full first responder status to grocery workers, and rules are inconsistent from state-to-state.
"We spend a lot of time talking about healthcare workers, and they are important, but we're missing a lot of the pieces of the puzzle if we don't look at non-health care workers exposure," Yang said. "Their voices are really not being heard. I thought it was important to get this published so government agencies and store owners could take note of this and see that they should be protecting their employees more."