Meat and poultry processing plants were a major source of COVID infections in the early stages of the pandemic, accounting for up to one in every 12 infections in surrounding areas, according to a new study.

The researchers, from the University of Chicago and Columbia University, found that by July 21, up to 310,000 COVID infections, with up to 5,200 deaths, were traceable to “proximity to livestock plants.” They concluded that residents of counties with meat processing plants were 51% more likely to get infected and 37% more likely to die, even with correction for risk factors.

In addition, plants that received USDA waivers on line speed maximums caused more infections, the study says.

Meat industry representatives told Bloomberg News that the study, which analyzed data only through July, did not take into account the improvement in COVID transmission due to improvements in working conditions since then.

The study was published in Proceedings of the National Academy of Sciences of the U.S.