(CNN) -- Shoppers who are vaccinated against Covid-19 can shop without masks in some Walmart, Sam's Club and Costco locations as well as at Trader Joe's, the companies said in separate announcements Friday.

The news follows new guidance that fully vaccinated people can go without masks under most circumstances.

On Thursday, the Centers for Disease Control and Prevention announced that fully vaccinated people don't have to wear masks or practice social distancing indoors or outdoors, except when in healthcare settings, on public transportation, or in other areas where governments require masks. People will still need to follow workplace and local businesses' mask guidance, the CDC said.

Friday afternoon, Walmart executives sent out a letter to Walmart and Sam's Club stores as well as supply chain facilities encouraging employees to get vaccinated. The letter noted that as of Friday, "vaccinated customers and members are welcome to shop without a mask," while unvaccinated shoppers are asked to keep wearing masks in stores.

Employees who are fully vaccinated will be permitted to work without masks starting May 18, the letter said, adding that some workers may have to continue to wear masks for health or sanitation reasons.

The letter added that "masks will also continue to be required by some city and state ordinances, and we will follow those requirements."

At Costco, vaccinated customers can shop without masks in US locations where there are no state or local mask mandates.

"We will allow members and guests who are fully vaccinated to enter Costco without a face mask or face shield," in those areas, the company said in a message posted to its website Friday. "Costco continues to recommend that all members and guests, especially those who are at higher risk, wear a mask or shield."

Costco won't require proof that customers have been vaccinated. Instead, the message said, "we ask for members' responsible and respectful cooperation with this revised policy."

All customers will still have to wear face coverings in Costco's pharmacy and other healthcare settings, the company specified. They will also have to wear them if they are in stores in locations where there are mask mandates.

At Trader Joe's, "we encourage customers to follow the guidance of health officials, including, as appropriate, CDC guidelines that advise customers who are fully vaccinated are not required to wear masks while shopping," the grocery chain said in a statement on its website Friday.

Trader Joe's is keeping many of its pandemic policies in place, such as wellness checks for employees and spacing people out within its stores, according to the company's statement. The grocery chain will not request or require proof of vaccination from its customers, Trader Joe's spokesperson Kenya Friend-Daniel told CNN Business in an email. She noted that employees still have to wear masks at this time.

Other retailers are leaving their policies unchanged for now. For example, Walgreens has decided to keep its mask policy in place after taking some time to review its requirements, according to spokesperson Emily Mekstan.

"As a destination for COVID vaccine and testing, we have decided to keep our current face covering policy in place for the time being," Mekstan told CNN Business in an email Friday.

On its Covid-19 FAQ site, last updated on May 6, the company said "customers are required to wear face covers before entering the store except where doing so would inhibit the individual's health or where the individual is under 2 years of age." Kroger and others are still requiring customers to wear masks in stores.

Unions representing grocery store workers and retail workers said Thursday stores should continue requiring customers to wear masks to protect workers. Marc Perrone, president of United Food and Commercial Workers' union, called the CDC guidance "confusing," adding that it "fails to consider how it will impact essential workers who face frequent exposure to individuals who are not vaccinated and refuse to wear masks."

Lisa LaBruno, senior executive vice president at the Retail Industry Leaders Association, said on Thursday the guidance "creates ambiguity" for retailers because the national guidance may be out of step with local orders.

"These conflicting positions put retailers and their employees in incredibly difficult situations," she said.

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