More and more jurisdictions around the United States – from Nantucket to Louisiana to Seattle – are requiring or recommending masks indoors for vaccinated people in reaction to the fast-spreading COVID-19 delta variant. 

With many areas struggling to reach significant levels of vaccination, case numbers in the U.S. have increased by a factor of five in the past month. And high-profile infections among vaccinated individuals are showing that although the coronavirus vaccines prevent nearly all hospitalization and death among those who take them, vaccinated people can still catch the virus and test positive.

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Though the number of U.S. coronavirus deaths has remained almost completely stable for the past month, officials in many areas are growing concerned about the potential for mutations that could create more variants. And they're moving toward reimplementing public health rules that were a staple at the height of the pandemic.

The only state so far to make such a move is Louisiana, which on Friday evening updated its virus guidance to recommend that everyone wear masks in public indoor areas. 

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"All people – vaccinated and unvaccinated – should wear face masks while indoors if six feet of physical distance cannot be maintained," the state said in a press release. 

"This should come as no surprise to anyone who has watched our case counts and hospitalizations continue to climb," Democrat Gov. John Bel Edwards said. 

The most recent jurisdiction to make such a move is Provincetown, Massachusetts, which Sunday night implemented an indoor mask mandate for all, according to Town Manager Alex Morse. That mandate will stay in effect until the town has a COVID positivity rate of less than 3% for five consecutive days. 

Provincetown's move followed another Massachusetts vacation town, Nantucket, which issued a face mask advisory last week. It asked "all residents and visitors, regardless of vaccination status, to use masks while indoors and at public locations when physical distance is not possible."

St. Louis, Missouri, meanwhile announced Friday that it is requiring masks for all people in indoor public spaces. And the mandate takes effect Monday. 

"We are relentlessly committed to making vaccinations more accessible and convenient. In the meantime, we need everyone, vaccinated or not, to wear masks in crowded indoor settings," St. Louis County Department of Public Health Acting Director Faisal Khan said. 

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On the West Coast, Los Angeles County earlier this month required all people to wear masks indoors. And at least 18 other counties in the state recommend that everyone, including the vaccinated, wear masks indoors, according to the Los Angeles Times. This includes nearly all of the Bay Area, the paper reported.

"After vaccination, masking is the next most powerful tool we have to protect ourselves and each other during this latest wave of infections," City of Berkeley Health Officer Lisa Hernandez said earlier this month. "Wearing masks, especially indoors and in crowded outdoor settings, will help us contain this more transmissible variant."

And on Friday, the top public health official for Seattle, Washington, and King County, where the city is located, made an official recommendation that all people wear masks indoors regardless of their vaccination status. 

"With the rise in #COVID19 cases in @KingCountyWA, Public Health – Seattle & King County now recommends that all residents five years of age and older, regardless of vaccination status, wear face coverings in indoor public settings," the Seattle and King County public health authority tweeted Friday. 

The delta variant is even affecting Congress itself. Democratic House chiefs of staff planned to hold their weekly meeting in-person Monday for the first time since the pandemic began. But they abruptly canceled what would have been a milestone event "out of an abundance of caution," partially due to breakthrough infections among vaccinated staffers. 

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Masking and mask mandates for the vaccinated – as the U.S. tries to increase vaccinations, what most agree is the best tool against the virus – are highly controversial among even medical professionals. 

Dr. Marc Siegel, Fox News medical contributor and NYU Langone Medical Center professor of medicine, said last week that there is "value" for a vaccinated person to wear a mask "in close quarters with a bunch of unvaccinated people," especially with the delta variant. But, he said, he is not a fan of mask mandates because they are "overused and I think that they make people feel bad and they make people feel punished."

Chief presidential medical adviser Dr. Anthony Fauci said on CNN Sunday that a recommendation for vaccinated Americans to mask up is under "active consideration" by federal authorities. If the Centers for Disease Control and Prevention (CDC) were to do that, it would spur many more city and state mandates. 

Former Surgeon General Jerome Adams is actively calling for more mask mandates. He blames the CDC decision to relax masking guidelines in May for the current virus surge. 

"The CDC could help by acknowledging that its prior messaging has not been effective and actually harmful," Adams wrote in the Washington Post. "Instead of vax it or mask it, people might need to vax it and mask it."

Dr. Marty Markary, a Fox News medical contributor and Johns Hopkins School of Medicine professor, is a vocal opponent of renewed mask mandates.

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People who are unvaccinated, Markary said on "Fox Report" Sunday, have had months to get the jab and therefore "have chosen not to get vaccinated at their own individual risk."

"Asking every American to change their lifestyle for their benefit is not something, I think, where there's a political will in some parts of the country," he added.

Fox News' Chad Pergram and the Associated Press contributed to this report.