ARLINGTON, Va. (AP) — Tyson Foods will require all of its U.S. employees to get vaccinated against COVID-19, becoming one of the first major employers of front-line workers to do so amid a resurgence of the virus.

Microsoft also announced Tuesday that it will require proof of vaccination for all employees, vendors and visitors to its U.S. offices starting in September, following similar actions recently taken by Google and Facebook. Microsoft also postponed its planned return to the workplace from September to no earlier than Oct. 4, although it will allow flexibility for some employees to continue working from home, including parents of children who are not eligible for vaccines.

While vaccination mandates have gained traction among employers, many of the companies adopting them have workforces that can transition to remote locations easily.

Many companies that rely on large low-income workforces have far largely declined to mandate vaccines for their front-line workers, making Tyson's announcement significant.

One of the world's largest food companies, Tyson said that members of its leadership team must be vaccinated by Sept. 24 and the rest of its office workers by Oct. 1. Its front-line workers must be vaccinated by Nov. 1, although the company said the specifics were being negotiated with unions.

Just under half of its U.S. workforce — about 56,000 employees — have been vaccinated after the company staged more than 100 vaccination events since February, it said. The Springfield, Arkansas, company plans to continue with those events and offer a $200 bonus for all front-line workers who receive a vaccine.

The United Food and Commercial Workers, which represents Tyson workers, criticized the company for imposing the requirement while the vaccines still have only emergency approval from the Food and Drug Administration. UFCW International President Marc Perrone said the union would be meeting with the company over the next weeks to "ensure that the rights of these workers are protected, and this policy is fairly implemented."

"While we support and encourage workers getting vaccinated against the COVID-19 virus, and have actively encouraged our members to do so, it is concerning that Tyson is implementing this mandate before the FDA has fully approved the vaccine," Perrone said in a prepared statement.

In a memo to employees, Tyson CEO Donnie King expressed alarm over the rise of the delta variant and made clear the vaccine requirement was needed to overcome persistent hesitancy to get the shots.

"We did not take this decision lightly. We have spent months encouraging our team members to get vaccinated – today, under half of our team members are," King wrote.

Tyson, whose brands include Jimmy Dean and Hillshire Farm, has grappled with deadly outbreaks of the virus at its plants and faced lawsuits from the families of some workers. Tyson said the number of infections at its plant are currently low after the company spent $700 million to better safeguard workers.

Other companies, including Amazon, Walmart and major grocery chains, have so far declined to mandate vaccines for their front-line workers, in part to avoid fueling a labor crunch and persistent worker turnover. The companies are instead continuing with information campaigns, bonuses, time off and other incentives.

Many unions also are firmly opposed to vaccine mandates for their workers.

The spread of the delta variant is also prompting some companies to reimpose mask mandates for workers — even those who are vaccinated — in keeping with new guidance from the Centers for Disease Control and Prevention.

Unionized auto workers at three companies — General Motors, Ford and Stellantis — will have to go back to wearing masks regardless of their vaccination status, according to a decision announced Tuesday by a task force of representatives from the companies and the United Auto Workers. The move comes just under a month after vaccinated union workers were allowed to shed their masks.

The task force encourages all workers to get vaccinations so mask requirements can eventually be relaxed.

However, Brian Rothenberg, spokesman for the 397,000-member United Auto Workers, said the union is against vaccine requirements because some people have religious or health concerns about vaccinations.

Still, tougher vaccine rules are gaining traction among restaurants, bars and some big entertainment companies, both for workers and customers. Some employers, including the federal government and some state and local authorities, are requiring that unvaccinated workers put up with weekly testing, stopping short of an outright mandate.

In Las Vegas, MGM Resorts International announced that unvaccinated employees would have to pay $15 to get tested for the virus onsite, or obtain a test offsite and bring in the results. The company also said that unvaccinated employees would not be paid for time off to quarantine if they test positive for the virus.

MGM Resorts has conducted several vaccination clinics and offered incentives including drawings for employees to win prizes such hotel stays and cash. But President and CEO Bill Hornbuckle expressed frustration at the region's low vaccination rate in a letter to employees imploring them to get the shots.

"Our region's low vaccination rate is putting us back on the path to overrun hospitals, unnecessary deaths, fewer tourists, and possible furloughs and layoffs," Hornbuckle wrote. "None of us want that."

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AP Auto Writer Tom Krisher in Detroit and AP Technology writer Matt O'Brien in Providence, Rhode Island, contributed to this story,