The new federal rule comes after many of the largest meatpacking plants have already vaccinated many of its employees.

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The Occupational Safety and Health Administration is set to publish a new rule on Friday that will require employers of more than a hundred workers to either require vaccinations or test everyone regularly and enforce mask-wearing. It includes meatpacking plants, which have been linked to tens of thousands of COVID-19 cases and hundreds of deaths.

The rule, what's called an "emergency temporary standard," is the kind of enforcement that advocates of meatpacking workers and unions representing plant employees had been asking for since the early days of the pandemic.

[Read more: The Trump Administration told an Illinois health department to leave Rochelle Foods alone. Then a second COVID-19 outbreak struck the plant.]

Companies have until Jan. 4 to either have workers vaccinated or have regular testing, according to Bloomberg News.

In 2020, the Trump administration decided to enforce very little in meatpacking plants. Once vaccines were available, companies implemented their own strategies — such as free meat giveaways — to encourage workers to get vaccinated.

When President Joe Biden took office earlier this year, he asked OSHA to identify ways it could better protect workers, but meatpacking plants were left out of an emergency standard issued earlier this year.

[Read more: ‘They think workers are like dogs.’ How pork plant execs sacrificed safety for profits.]

The new standard comes after at least 400 plant employees have died from complications from COVID-19. Many workers have also already been vaccinated, at least at the largest meatpacking companies. Almost all Tyson Foods employees have gotten the shot, according to the company, and JBS said about three-quarters of its workforce has been vaccinated.

Below is the unpublished emergency temporary standard.