More than a dozen states quickly embraced new federal guidelines that say fully vaccinated Americans no longer need to wear masks indoors or out in most cases. But other states and cities and some major businesses hesitated amid doubts about whether the approach is safe or even workable.

As many business owners pointed out, there is no easy way to determine who has been vaccinated and who hasn’t. And the new guidelines, issued Thursday by the Centers for Disease Control and Prevention, essentially work on the honor system, leaving it up to people to do the right thing.

Labor groups and others warned that employees at stores, restaurants, bars and other businesses could be left exposed to the coronavirus from customers and could be forced into the unwanted role of “vaccination police.”

But in Malvern, Pennsylvania, owner Sean Weinberg took down the mask signs Friday at Restaurant Alba, which he runs with his wife. He also emailed his employees to let them know they can forgo masks at work if they are fully vaccinated.

“It’s just a headache we don’t want to have to fight any more,” Weinberg said.

Several major chains, including CVS, Home Depot, Macy’s and supermarket giant Kroger Co., said they are still requiring masks in stores for the time being, though some said they are reviewing their policies.

But Walmart, the world’s largest retailer, Costco and Trader Joe’s said Friday that they won’t require vaccinated shoppers to wear a mask in U.S. stores, unless state or local laws say otherwise.

Vaccinated shoppers can go maskless immediately, Walmart said. Vaccinated workers can stop wearing them on May 18. As an incentive, the company said it is offering workers $75 if they prove they have been vaccinated.

Both Costco and Trader Joe’s said they would not require proof of vaccination, but employees at the grocery chain will still need to cover their faces.

Half the states had mask requirements in place for most indoor spaces when the CDC issued its recommendations amid tumbling cases and rising vaccination rates.

Nearly 47% of the U.S. population has received at least one dose of COVID-19 vaccine, and cases have dropped to their lowest level since September, at an average of about 35,000 a day. CDC Director Dr. Rochelle Walensky noted in making the announcement that the vaccine has proved powerfully effective in preventing serious COVID-19 illness.

Michigan, Nevada, Oregon, Ohio, Virginia, Delaware, Maryland, Pennsylvania, Minnesota, Kentucky, Washington, Maine, Vermont, Connecticut, North Carolina, Kansas, Colorado and Rhode Island announced plans to fall in line with the CDC guidance either immediately or in the coming weeks. Some cities, including New Orleans and Anchorage, did the same.

Oregon Gov. Kate Brown said the new approach makes clear that vaccines are the fastest way to get back to doing the things “we all love.” Kentucky Gov. Andy Beshear called the guidance a “game-changer.” And Washington Gov. Jay Inslee said the change is “a heck of a benefit.”

Other states, such as California, Wisconsin, New Jersey, Hawaii and Massachusetts, and cities like Minneapolis and St. Paul kept mask rules in place for the time being.

“We’re frankly not there yet,” New Jersey Gov. Phil Murphy said. And Hawaii Gov. David Ige said, “We are unable to determine who is vaccinated and who is not vaccinated. The best mitigation measure is for everyone to wear a mask.”

Industry leaders warned of the potential for confusion and hard feelings among customers because of the varying rules from place to place. Even in states that have dropped mask mandates, stores and other businesses can still require face coverings if they want.

Confusion over the guidance extended to the White House, where press secretary Jen Psaki said, “I think we’re still figuring out how to implement it.”

The CDC and the Biden administration had faced pressure to ease restrictions on fully vaccinated people in part to highlight the benefits of the shots and motivate other people to get inoculated.

Restaurant workers in places where mask mandates remain are finding themselves caught in the middle, said Jot Condie, the president of the California Restaurant Association. He said his phone has been “blowing up” with reports of increasingly belligerent customers.

“The person who is not wearing a mask will say, ‘My president just told me that the CDC just issued guidance and I’ve been vaccinated and I’m not going to wear a mask,’” he said.

Scott Dolch, executive director of the Connecticut Restaurant Association, said owners are concerned about being put in a difficult position.

“They’re like, OK, now I have to deal with the honor system, hoping that that person that told me they’re totally vaccinated” is telling the truth, Dolch said.

The CDC announcement sent airline stocks soaring, though the guidance still calls for masks in crowded indoor settings such as planes, buses, trains, hospitals, prisons and homeless shelters, and says people should obey all local and state regulations.

Republican Rep. Dusty Johnson, South Dakota’s lone congressman, marked the announcement by sharing a video demonstrating how cast-off masks can now be used for things like suit pocket handkerchiefs, bookmarks or beer cozies.

“It seems too wasteful to just throw them away,” he said. “I think I’ll have my mother make them into a quilt.”

Shelby Lofton, a reporter for WKYT-TV in Lexington, Kentucky, tweeted: “So, I guess I’ll start wearing lipstick again. Also need to work on my poker face.”

In Detroit, a fully vaccinated Christoph Cunningham, 28, wore a mask as he rode an electric scooter to a bar for lunch and said he agrees with the relaxed guidelines.

“I have confidence in the science behind it all,” said Cunningham, who runs a catering business. “I’ll eventually take my mask off more and more.”

The president of the American Federation of Teachers, Randi Weingarten, said the 1.7 million-member union is still trying to sort out what the change means for schools. Many school districts already ditched mask requirements in recent weeks, as had many states and cities, as virus numbers fell.

That meant the CDC announcement didn’t mean much in places like the tourist town of Branson, Missouri, which dropped its mandate early last month after several mask supporters were voted out of office.

“I think it just further supports the decision we made to lift the mask mandate,” said the town’s new mayor, Larry Milton. “It was dividing our community. We heard loud and clear from voters that they wanted the mask mandate repealed.”

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Associated Press writers Sara Burnett in Chicago, Ed White in Detroit, Zeke Miller in Washington and Rachel La Corte in Olympia, Washington, contributed to this report.