A shocking new study published in the journal Occupational and Environmental Medicine found that 20% of grocery workers became a significant transmission risk to customers because they not only tested positive for the virus, but most did not have any symptoms.

The researchers tested 104 employees at a Boston grocery store in May with nasal swabs and found one-fifth tested positive for COVID-19. According to the New York Post, the 20% infection rate is much higher than found in other areas of the community and the workers who dealt directly with customers were 5 times as likely to test positive for the disease. Furthermore, three out of four of confirmed cases had no symptoms.

“This is definitely very alarming as it means retail grocery store employees are exposed to customers and sort of serve as a middleman for the virus—like a superspreader almost,” said Dr. Justin Yang, at the Harvard T.H. Chan School of Public Health, who was part of the study team, according to the Post.

According to CNN, the workers tried to take precautions and 91% of them wore face masks. However, only 66% said they were able to safely social distance on the job.

“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,” said Yang. According to the United Food and Commercial Workers International Union, at least 108 grocery works have died and more than 16,300 have been infected or exposed to the virus.