Smithfield Foods pork plant, the world’s biggest pork processor, sits closed indefinitely due to a rash of coronavirus cases among employees as the spread of the coronavirus disease (COVID-19) continues, in Sioux Falls, South Dakota, U.S., April 17, 2020.

Meatpacking plants responsible for 10% of beef production and 25% of pork production have been affected by closures amid the coronavirus pandemic, the United Food and Commercial Workers International Union said Thursday.

As the coronavirus spreads among meatpacking workers who often work in proximity, concerns are growing about potential meat shortages — and the nation's overall food supply.

In a letter to Vice President Mike Pence dated Thursday, the UFCW asked the White House coronavirus task force, which Pence leads, to strengthen national safety standards in meat processing plants. The union's recommendations include halting line speed waivers recently granted by the U.S. Department of Agriculture that allow plants to operate at faster speeds.

"When they have to start closing these plants down or they have to spread the lines out for social distancing or slow them down in some ways, which we're calling for as it relates to these line speed waivers, it will in fact create some shortages in the stores," UFCW President Marc Perrone said on a press conference call Thursday. "I don't think there's any doubt about it."

The union, which represents 80% of beef and pork production workers, said that 13 packing and food processing workers have died after contracting Covid-19 and 5,000 meatpacking workers have tested positive or been exposed. Since the pandemic began more than 2.6 million cases have been identified worldwide, and at least 186,372 people have died, according to Johns Hopkins University research.

"Social distancing is hard and sometimes impossible in some areas of the plant," Itzel Goytia, a beef plant worker at a Cargill facility in Dodge City, Kansas, said on the union's conference call.

Tyson Foods said Wednesday it would indefinitely suspend operations at the company's largest pork plant. The announcement follows Smithfield Foods' notice last week that it would close its Sioux Falls, South Dakota, facility after 230 workers tested positive for the virus.

Such closures can cause a domino effect. The stoppage at Smithfield's pork plant, one of the largest in the country, forced the closure of a Missouri ham plant that receives raw materials from the Sioux Falls facility.

Some meatpacking plants that closed due to outbreaks have reopened. An Iowa facility run by National Beef resumed production Monday after two weeks idle and 177 positive tests for coronavirus.

Mark Lauritsen, UFCW's director of food processing, meatpacking and manufacturing, said on the conference call that shortages could persist post-crisis because hog and cattle farmers will raise smaller herds to increase prices.