• Local officials in California and Nevada last week advised all residents, including vaccinated people, to start wearing masks in public settings indoors.
• Leaders in at least three more states have either reinstated mask mandates, issued facial covering recommendations or threatened the return of strict public health limits.
• Covid cases jumped by 55% over the last week to an average of 37,000 over the last seven days, according to a CNBC analysis of data compiled by Johns Hopkins University.

From Los Angeles to Massachusetts, local officials across the country are pleading with Americans to once again wear masks inside as the delta variant rips through the U.S.

Several counties across California and Nevada are now advising all residents to wear masks in public indoor settings — whether they are vaccinated or not. Local leaders in at least three more states have reinstated mask mandates, issued facial covering recommendations or threatened the return of strict public health limits for all residents — in defiance of federal health guidelines that say vaccinated people don't have to follow those protocols in most settings.

"A rise in case counts was not unexpected as the community began to fully reopen," Southern Nevada Health District spokeswoman Jennifer Sizemore told CNBC in an email. Home to Las Vegas, Nevada's Clark County tightened its mask recommendation last week after Covid-19 cases and deaths surged by 50% over the previous week. In total, 4,599 new infections were reported last week along with 33 coronavirus-related deaths, according to data compiled by the Centers for Disease Control and Prevention.

After months of falling cases, Covid infections are on the rise again in the U.S. New cases jumped by 55% since last week to an average of 37,000 new cases per day over the last seven days, according to a CNBC analysis of data compiled by Johns Hopkins University.

The CDC on May 13 eased its Covid guidelines on masks for fully vaccinated people, saying they didn't need to use them or practice social distancing in most settings. CDC director Dr. Rochelle Walensky told lawmakers at a Senate hearing Tuesday that the agency is actively reviewing its mask and other public health guidelines as the virus and pandemic evolve, especially as scientists learn more about the delta variant and how vaccines hold up against it.

"A lot has changed since May 13," Walensky testified. "We now have a variant circulating in this country that at the time was 3% (of new cases) and is now 83% and much more transmissible."

The delta variant is spreading throughout the country, particularly in areas with low vaccination rates, she said. Nearly two-thirds of the counties in the U.S. have vaccinated less than 40% of their residents, "allowing for the emergence and rapid spread of the highly transmissible delta variant," and that's leading to a rise in hospitalizations and deaths, she said.

That's starting to show in Nevada, which has fully vaccinated just 43.5% of its population, according to CDC data. Clark County recorded 641 new Covid hospitalizations last week, 23% more admissions than in the previous seven days. Despite the resurging outbreak in the Las Vegas area, Sizemore said the county's vaccination rate has hovered just under 42% for the last two weeks.

"However, the community's vaccination rate has been slowing and unvaccinated people are not taking recommended precautions, including wearing masks and continuing to practice social distancing," Sizemore said.

Nevada isn't the only state to reinforce its mask guidelines. On Friday, seven counties in California's Bay Area recommended using masks indoors, stopping short of a full mandate. The city of Berkeley, California, similarly called for the continued use of masks.

Further south, Los Angeles County restored its mask mandate for public indoor places on Saturday. The county initially eliminated the mandate on Thursday, when the state formally rolled back a series of executive measures designed to mitigate against the spread of Covid.

White House chief medical advisor Dr. Anthony Fauci said Los Angeles County's new mask mandate could serve as a prototype for other regions that have high infections rates. He said he expected schools and businesses to continue enforcing their own mask guidance to protect against the delta variant.

"If you want to go the extra mile of safety even though you're vaccinated, when you're indoors, particularly in crowded places, you might want to consider wearing a mask," Fauci said in an interview with CNBC's "Closing Bell" on Wednesday.

In Massachusetts, officials in Provincetown advised all individuals Monday to resume wearing masks indoors after Fourth of July celebrations led to an outbreak of new cases.

In Orleans Parish, Louisiana — where the CDC reported 560 new coronavirus cases last week — New Orleans Mayor LaToya Cantrell authorized an indoor mask advisory on Wednesday to curtail the spread of the delta variant. And New Jersey Gov. Phil Murphy told CNBC's Squawk Box on Tuesday that he wanted to avoid reinstating a mask mandate, pushing instead for residents to get vaccinated.

"For the time being, I hope we don't have to do that," Murphy said. "If we have to, we will."