All around Sioux City, Iowa, on Friday, restaurants were opening their doors to socially distanced customers. Treadmills were starting to roll as Iowans unleashed their quarantined frustrations at the fitness center. Shopping centers echoed with the footfall of patrons for the first time in weeks.

But not so in the city itself, home to the fastest-growing coronavirus infection rate in the country.

“Now’s not the time,” said Bob Scott, Sioux City’s mayor. “We’re not even close to being ready.”

As America’s piecemeal reopening gathers pace, states are making their own choices about how quickly to move toward what passes for the new normal in the age of covid-19. Numerous states joined the ranks Friday of those that have eased restrictions as April’s stay-at-home orders expired.

But within many states, there are still divisions: Even as rural and suburban areas reopen, cities are staying firmly shut.

The approach reflects the uneven toll of a virus that is drawn to density and has attacked urban centers with abandon, even as more-sparsely populated areas have been spared.

The advantage of staggered reopenings that take into account those differences is seductively clear: By opting for a “targeted approach,” Iowa Gov. Kim Reynolds (R) argued this week in announcing the reopening of 77 of the state’s 99 counties, hard-hit places can stay hunkered down as those with low or declining infection rates swing back to life.

“I shouldn’t punish half of the state when we’ve got a significant spike in eight areas,” Reynolds said.

Yet experts say that opening some places and not others carries its own peril: The approach could make it more difficult to enforce social distancing in cities and more likely that the coronavirus will continue its spread into the countryside.

“People want to go to restaurants. They want to visit their friends. They want to travel,” said Nadia Abuelezam, a Boston College epidemiologist. “So how do you convince people that if you live on one side of a line, you can’t leave your home, but if you live on the other, you can?”

Scott worries about exactly that — and more. His northwest Iowa city has rapidly become a coronavirus hot spot — case numbers were doubling every two or three days this week — as meatpacking workers have fallen ill. The nearby Iowa countryside, by contrast, has been relatively unaffected.

But as those surrounding counties open up — even as his city of 80,000 stays closed — Scott worries his residents could inadvertently spread the virus beyond city lines.

“After seven weeks of being cooped up, people are going to want to go out,” he said. “And they’re going to want to go to those counties where the restaurants are open.”

Despite the drawbacks, the staggered approach to reopening across a state is one that has been embraced by a number of governors, Republican and Democrat alike.

Iowa’s neighbor Missouri is doing it. Both states are Republican-run. Colorado, led by a Democrat, Gov. Jared Polis, allowed personal training and dog grooming to resume this week — but not in Denver, where stay-at-home orders have been extended until May 8. When New York starts to reopen, Gov. Andrew M. Cuomo (D) has said it will be in the rural upstate, well before downstate New York City gets the chance.

Other states have implemented a more uniform approach. Texas Gov. Greg Abbott (R), for instance, has insisted that his loosening of restrictions on nonessential businesses be applied statewide, despite protests from leaders in cities such as Dallas and Austin who have argued the moves are premature.

Even when cities have a choice, some mayors have found the idea of staying closed unappealing if the rest of the state is opening up.

In Oklahoma, cities had the option to keep their stay-at-home orders in effect as the governor lifted statewide rules. Tulsa Mayor G.T. Bynum, a Republican, was torn by the choice.

On the one hand, the city of 400,000 was not meeting a critical prerequisite for easing social distancing: a 14-day decline in new cases. Numbers were, in fact, still rising locally, even as they fell statewide. Bynum’s health director said it wasn’t time. Bynum agreed.

But with the rest of the state due to resume much of its normal economic activity, the mayor decided that resistance was futile.

“There’s not a glass dome over Tulsa protecting us. We don’t exist within a bubble,” said Bynum, who had shut down the city earlier than the state. “We would be asking people to keep making sacrifices in the name of public health when people in other communities are not and continue to spread the virus into our community.”

Bynum said he “hated” the choice and believes that case numbers will rise — Tulsa County’s case count stood at just over 580 as of Friday. But he hopes the spike can be mitigated if reopened businesses rigorously enforce social distancing, including requirements that customers stay apart from one another.

“This isn’t a celebration. We can’t go back to life as it was,” Bynum said as the city prepared for a Friday relaunch of restaurants and other businesses.

Even a limited reopening is still out of reach in St. Louis, where the city and the adjacent counties account for some two-thirds of Missouri’s more than 7,500 cases.

Most businesses statewide will be permitted by Gov. Mike Parson (R) to reopen on Monday, but not in St. Louis or the surrounding county. Mayor Lyda Krewson (D) said that the reopening of her city of 300,000 will have to wait until case numbers have had sufficient time to fall from their peak, and that she and the governor agreed it made sense to pursue different approaches.

“Clearly one size doesn’t fit all,” she said. “We’re in a different situation than other areas are.”

Krewson said her city’s outbreak had disproportionately struck poorer and predominantly African American neighborhoods. In those areas — and across the city — the question of when it is safe to reopen is a wrenching one, pitting economic needs against public health.

“People want to open back up. They’re also afraid of it,” she said. “No one wants a second wave of covid.”

Up the Missouri River, in Sioux City, the first wave is still proving tenacious. The surrounding county had just 158 cases of coronavirus as of last Thursday. A week later, the total was up to 851, a more than fivefold increase, as clusters of infection broke out among workers at area meatpacking plants.

“It exploded overnight,” said Scott, the mayor.

He said he spoke with Reynolds before she announced her reopening plans and urged her to keep his county closed. Most residents and business owners, he said, have been supportive — despite the economic pain of staying shut for at least several more weeks.

“Most businesses recognize that we’ve got a problem,” Scott said. “By rushing to reopen, we could make it worse.”

It is a worry that is shared 20 miles down the road, in the small city of Le Mars, Iowa, where the surrounding county has had only 12 confirmed cases. Mayor Dick Kirchoff said there is inevitable concern that the number could jump as restaurants reopened Friday and Sioux City residents are tempted to come visit.

But Kirchoff said he trusted business owners to reopen responsibly. “They know what the rules are, and they’re preparing to follow them,” he said.

And at least at first, caution will be a powerful ally. “People are not going to be knocking down the doors to get in,” he said.

Yet experts fear the restraint may not last. Abuelezam, the epidemiologist, noted there will be a significant lag between when states reopen and when signs of a renewed outbreak emerge. In the meantime, people could become bolder in embracing risk — and thereby accelerate the spread.

That is just one reason Des Moines Mayor Frank Cownie (D) said he would have preferred to see the entire state — not just cities — stay closed. His city had the nation’s fifth-fastest-growing infection rate as of Friday, and he noted that other Iowa outbreaks were also rising fast. Without a coordinated response, he said, he worried it will be extremely difficult — if not impossible — to control the spread.

“We’re in the middle of a massive uptrend in the number of cases not only in our county but across the state,” Cownie said. “We have to be very, very careful moving forward.”