(CNN) A few states this weekend are running a tricky experiment: Trying to reopen certain businesses and services amid the coronavirus pandemic , even against the advice of some health experts who say it risks dangerously spiking the number of cases.

In Georgia, Tammy Noboa has seized on her state's blessing to open her hair salon after weeks of closure -- and she says deciding to do so wasn't hard.

"I have to work. I've got bills to pay," said Noboa, who accepted seven appointments Saturday at her newly reopened Dominican Hair Salon in Douglasville.

Georgia is one of the states that allowed some businesses to reopen Friday, weeks after shutting them down to help prevent the spread of coronavirus.

More than 53,600 Covid-19 deaths have been reported in the US so far, with more than 200,000 deaths reported worldwide, according to Johns Hopkins University . The US has reported more than 938,600 cases of the disease.

Georgia businesses allowed to reopen include places where clients and workers get close: barber shops and hair salons, tattoo parlors, gyms and bowling alleys -- with some guidelines for social distancing and sanitation.

In Oklahoma, salons, barbershops, spas and pet groomers took appointments Friday, and some state parks and outdoor recreation areas also reopened.

The states' moves run counter to the advice of experts who've run a University of Washington model suggesting no state should reopen their economies before May 1 and many should wait longer.

Georgia should not begin to reopen until at least June 22, according to those behind the model at the university's Institute for Health Metrics and Evaluation.

In the Atlanta-area city of Brookhaven, Mayor John Ernst would rather nonessential businesses stay closed until his state proves it meets federal guidelines , calling for milestones like a 14-day downward trend in coronavirus cases.

"Even the (business owners) who open up say, 'I don't know if I'm doing the right thing," Ernst said Saturday. "(Reopening) needs to be an orderly process."

Atlanta Mayor Keisha Lance Bottoms bluntly told CNN's Wolf Blitzer on Friday evening that people should ignore the governor's plan, saying, "Stay home, nothing has changed."

Georgia and Oklahoma aren't alone

Besides Georgia and Oklahoma, these states also are easing restrictions:

• Alaska allowed salons and restaurants to open in many areas Friday, though restaurants must keep distance between tables and can't exceed 25% of their normal capacity.

Texas on Friday allowed retail stores to make curbside sales.

• Michigan Gov. Gretchen Whitmer allowed some businesses -- landscapers, lawn-service companies, plant nurseries and bike repair shops -- to reopen Friday, subject to social-distancing rules.

• In Iowa, elective surgeries and farmers markets can reopen on Monday.

• In Tennessee, restaurants can reopen Monday at 50% capacity. Retail stores may reopen Wednesday under the same guideline, state Gov. Bill Lee said.

Colorado, Minnesota and Montana also will ease restrictions in varying degrees in the coming week. And other governors are setting dates for when their reopening plans will kick into action.

Yet other leaders have stopped short of setting a timeline. In San Francisco, which issued the country's first sweeping stay-at-home order in mid-March, Mayor London Breed said the order is "very likely" to be extended for a few more weeks past May 3.

"How we reopen is going to be important to ensuring that we do it responsibly so that we don't go backwards," said Breed, who stressed the importance of having enough PPE, testing and requirements for social distancing.

Even some states without reopening plans have decisions to make: More than 10 have stay-at-home orders expiring by the end of next week , though they can be extended.

Business owners struggle with reopening decisions

Some business owners in Georgia told CNN they felt wary of reopening, but they did so to pay their bills.

"I'm at the point where I have to do something ... I'm about to lose my business if I don't," said Tim Timmons, owner of Salon Gloss in Woodstock, on Friday.

Hairstylists and barbers in Georgia wore masks and gloves when people arrived for trims and hair colors Friday.

Timmons said he put measures in place to guard against the spread of the virus. The salon wasn't running on full staff, and employees stood 14 feet apart. Customers had their temperatures taken when they arrived and were asked if they've come into contact with anyone who's had the virus.

But other owners said now wasn't the time to reopen.

"I said, 'No, absolutely not. Get your hair done for what?'" Sabrina Watkins said of her hair salon in College Park, an Atlanta suburb. "There's a pandemic, people are dying. As much as I love the business, now is not the time, regardless of who says it is."

"No way we're opening back up," said Zeb Stevenson, chef at Atlanta restaurant Redbird. "I don't think it's safe. If we reopened and then had to close back down again it would be a grave situation for the business. Better to wait it out and use science and common sense as a guide."

Lequawn James, an Atlanta nurse practitioner and bodybuilder, said Saturday he would not yet visit any reopened gyms.

JUST WATCHED Hear from Georgia business owners as state starts reopening Replay More Videos ... MUST WATCH Hear from Georgia business owners as state starts reopening 02:56

He survived coronavirus after spending 10 days in intensive care . And he's exercising, but alone, with equipment that he's put into a rented storage unit.

He said he understands workers' struggles. But he thinks it's too soon to work at or exercise at places like gyms.

"I know money is what people need to survive, but you may not be around to spend it if you contract this virus," he said.

At a bowling alley in Douglasville, a suburb east of Atlanta, Leon Perpignan was in line 10 minutes before it opened Friday at noon. Typically, he bowls four times a week, he said. About a dozen bowlers were there shortly after opening.

"I know a lot of people disagree and say they should have waited," he said, "but I was 100% ready (for this)."

"Besides," he added, "all my 'honey-do' lists are done."

Southern California beaches are busy

In Southern California, the social distancing challenge involved leisure, not business.

Crowds descended on Newport Beach as a heat wave hit the region, but most beachgoers appeared to be keeping their distance from each other, police and lifeguards said.

Surfer John Ton observed the crowd and said, "I think people should stay home unless they're surfing or exercising or something, and I think it's weird people from out of the state or county are coming."

Beaches in Los Angeles and San Diego counties were closed this weekend, but other beaches were open.

Hawaii has relaxed beach restrictions, Gov. David Ige said during a Saturday news conference.

Beaches are now open for exercise like jogging, running or walking but people cannot loiter on the beach and must maintain social distance, Ige said. Groups of two people or more are now allowed to fish for subsistence or commercial purposed, Ige said. A previous restriction was two or less.

Ige said Hawaii has seen a 98% decrease in travelers arriving at airports since the 14-day quarantine rule was put in place.

No evidence yet on immunity from a 2nd infection, WHO warns

The World Health Organization is warning that it's too early to say whether people who have had Covid-19 are necessarily immune from a second infection. It is urging governments to not yet issue any kind of "immunity certificate" to people who had the disease.

"There is no evidence yet that people who have had Covid-19 will not get a second infection," the WHO said in a scientific brief Friday.

The WHO published the brief as guidance on how to adjust public health and social measures for the next phases of the Covid-19 response. The health agency said it is reviewing evidence on antibody responses to the novel coronavirus. The brief says "most" studies show that people who have "recovered from infection have antibodies to the virus."

But as of Friday, no study has "evaluated whether the presence of antibodies to (the virus) confers immunity to subsequent infection by this virus in humans," the WHO brief says.

The US Food and Drug Administration has now authorized three new coronavirus antibody tests, bringing the total number of FDA-authorized tests to seven.

The tests were approved under emergency-use authorizations, a lower regulatory standard used when the FDA believes a test's benefits could outweigh any risks.

On Saturday, the CEO of a group helping lead the vaccine effort said it might be necessary to start manufacturing coronavirus vaccines even before they have been fully tested to see if they can protect people from infection.

Manufacturing could begin even while some of the Covid-19 vaccines are in the first phase of human clinical testing, which is designed to demonstrate only safety, said Dr. Richard Hatchett of the Coalition for Epidemic Preparedness Innovations.

This plan could cut time without cutting corners or sacrificing efficacy or safety, he said on a National Academy of Sciences Covid-19 Update webcast.