(Bloomberg) -- A new study ties 6% to 8% of U.S. Covid-19 cases through late July to outbreaks at meatpacking plants and subsequent spread in surrounding communities.

The findings show “a strong positive relationship” between meatpacking plants and “local community transmission,” suggesting the plants act as “transmission vectors” and “accelerate the spread of the virus,” according to a peer-reviewed study published last week in the Proceedings of the National Academy of Sciences of the U.S.

The conclusions are sure to further inflame controversy over the role of the meatpacking industry in the pandemic and the Trump administration’s enforcement of workplace safety laws as outbreaks at slaughterhouses emerged. Trump issued an executive order on April 28 directing meatpackers to reopen closed facilities.

Researchers at Columbia University’s School of International and Public Affairs and the University of Chicago’s Booth School of Business found that the risk of excess death primarily came from large meatpacking plants operated by industry giants. Communities that acted to shut down slaughterhouses reduced spread, according to the researchers.

Overall, the researchers found 236,000 to 310,000 Covid-19 cases through July 21 associated with “proximity to livestock plants,” comprising 6% to 8% of virus cases at the time. Between 4,300 and 5,200 Covid-19 deaths were tied to being near meatpacking plants, representing about 3% to 4% of U.S. deaths in that time period.

“The vast majority” of those cases were “likely related to community spread outside these plants,” the researchers wrote.

The researchers also found plants that received waivers from the U.S. Department of Agriculture to increase their production-line speeds had relatively more county-wide cases. Earlier this month, the U.S. Department of Agriculture submitted a proposal to raise maximum line speeds nationwide for chicken-processing plants.

“Ensuring both public health and robust essential supply chains may require an increase in meatpacking oversight and potentially a shift toward more decentralized, smaller-scale meat production,” the study concluded.

Press representatives from the USDA and the North American Meat Institute didn’t immediately respond to requests for comment.

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