New federal guidance on mask-wearing is putting businesses and local officials in a tough spot.

In a dramatic about-face, the Centers for Disease Control and Prevention (CDC) advised Thursday that fully vaccinated Americans don't have to wear masks or practice physical distancing in most settings.

"We have all longed for this moment, when we can get back to some sense of normalcy," CDC Director Rochelle Walensky said during a White House news conference.

But the announcement caught many off guard, and raised a host of questions about how new policies would be implemented. States and cities were left scrambling to figure out whether to change their own rules to match the new federal advice.

Health experts, business leaders and labor groups said the new recommendations are too ambiguous, and warned that a lack of consistency across different sectors is concerning and dangerous.

There is no way to tell who is vaccinated and who is not without asking for proof. The federal government has ruled out any sort of national "passport," instead leaving it up to individual businesses.  

"Millions of Americans are doing the right thing and getting vaccinated, but essential workers are still forced to play mask police for shoppers who are unvaccinated and refuse to follow local COVID safety measures. Are they now supposed to become the vaccination police?" Marc Perrone, president of the United Food and Commercial Workers union, said in a statement.

Perrone criticized the CDC for failing to take into account the essential workers who face frequent exposure to individuals who are not vaccinated and refuse to wear masks.

“With so many states already ending their mask mandates, this new CDC guidance must do more to acknowledge the real and daily challenge these workers and the American people still face," he said.

Lawrence Gostin, a professor of global health law at Georgetown University, said it's too much of a burden to expect businesses to try and require proof of vaccination without any kind of technical guidance or support.

"I think particularly with a small- or medium-size employer, you can't expect them to figure out how to do a vaccine certificate system," Gostin said.

"The CDC is telling the public and the private sector that they have to make a very sharp differentiation between vaccinated and unvaccinated people, and yet the federal government stubbornly refuses to help businesses to gain proof of vaccination status," he added.

Even large retailers could be placed in a difficult position, either to enforce an existing mask mandate or to check for customers' vaccination status.

Walmart on Friday sent a memo to associates that fully vaccinated customers can shop without a mask effectively immediately, and that fully vaccinated associates will not have to wear a mask starting Tuesday. The mega-store did not specify how they will check if a customer or employee is vaccinated other than saying, “we will continue to request that non-vaccinated customers and members wear face coverings in our stores and clubs. We will update the signage in our facilities to reflect this.”

The Retail Industry Leaders Association (RILA), which represents major companies from CVS to Home Depot, said that firms want consistency on mask guidance, and that the safety of associates is a top priority.

The CDC announcement on masks "creates ambiguity for retailers because it fails to fully align with state and local orders. Retailers are reviewing the latest CDC guidance to determine if any changes should be made to their current safety protocols,” said Lisa LaBruno, senior executive vice president of retail operations and innovation.

LaBruno called on customers to keep in mind that the CDC makes guidance, not law, and there are still mask mandates in place at the state and local level.

“These conflicting positions put retailers and their employees in incredibly difficult situations. We urge state and local governments to coordinate with the CDC as additional guidance is issued on the road to normalcy,” LaBruno said.

She added that “frontline workers deserve this respect” from customers and guests following store protocols such as wearing a mask.

Prior to the announcement, about two dozen states mandated masking in public. But within hours, governors in states including Connecticut, Pennsylvania, Vermont and Washington promptly lifted their mask mandates.

Others, including in Massachusetts, California, New Jersey and New York, said they would wait.

"We know what's happened throughout the pandemic when the federal government just took a hands-off approach. You have inconsistency, haphazardness and confusion," Gostin said.

Trader Joe’s became the first grocery chain on Friday to announce they will allow fully vaccinated customers to not wear a mask while shopping. Like Walmart, the company did not specify any process to check if a customer without a mask on is vaccinated.

Other major retailers have said they will stick with their own coronavirus mitigation measures, following incidents of altercations with store and restaurant employees over the past year.

“Target will continue to require all of our coronavirus safety measures in all stores, including masks and social distancing, while we review updated guidance from the CDC and re-evaluate the guidance we offer our team and guests,” a spokesperson told The Hill.

Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, said he thinks companies may find it easier to keep their own mask rules in place.

"Most businesses are not going to change their policy for customers. Because you can't expect someone who works at a gas station or a convenience store or grocery store to go check people's vaccine status," Adalja said.

The National Retail Federation (NRF) said stores will follow state laws, but overall welcomed the new guidance as a path towards reopening the economy.

“The CDC’s updated guidance for fully vaccinated individuals will help to open the economy and get more people back to work. Retailers will continue to follow coronavirus-related laws and regulations governing store operations in each state,” Matthew Shay, CEO of the NRF, told The Hill.

The new guidance may also pose challenges for industries hit hard by the pandemic that have struggled to attract enough workers to meet rising consumer demand. Restaurants, bars, retailers, and ride share companies have all reported difficulties rehiring staff with the U.S. economy kicking back into pre-pandemic activities.

While Republican lawmakers and business groups have blamed expanded unemployment benefits for disincentivizing work, many jobseekers say they remain uncomfortable returning to close-contact jobs with just over a third of U.S. adults fully vaccinated.

The National Restaurant Association said it’s encouraged by the CDC’s decision because it could help the industry move towards being fully reopened. But, the group noted that operators have the option of determining how to enforce the new CDC guidance.

“Because restaurants welcome people who are both vaccinated and not fully vaccinated, operators will still need to work with their state and local regulators to ensure they are in line with all other mandates in place,” said Larry Lynch, the association’s senior vice president of science and industry.