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Dallas, Houston, Southeast Florida’s Gold Coast, the entire state of Alabama and several other places in the South that have been rapidly reopening their economies are in danger of a second wave of coronavirus infections over the next four weeks, according to a research team that uses cellphone data to track social mobility and forecast the trajectory of the pandemic.

The model, developed by ­PolicyLab at Children’s Hospital of Philadelphia and updated Wednesday with new data, suggests that most communities in the United States should be able to avoid a second spike in the near term if residents are careful to maintain social distancing even as businesses open up and restrictions are eased.

But the risk for resurgence is high in some parts of the country, especially in places where cases are already rising fast, including the counties of Crawford, Iowa; Colfax, Neb.; and Texas, Okla. and the city of Richmond. Since May 3, Crawford County’s caseload has risen by 750 percent, and Colfax County’s has increased 1,390 percent, according to state data compiled by The Washington Post.

This is an anxious moment for the nation as people emerge from shutdowns and communities try to reinvigorate economic activity. Scientists and public health experts are monitoring rates of infections and hospitalizations, but it is difficult to forecast during this transitional period because models struggle to capture how people actually behave, including adherence to social distancing and hand-washing practices.

There are preliminary signs, however, that hot spots — new clusters of coronavirus spread — could soon flare across parts of the South and Midwest.

“As communities reopen, we’re starting to detect evidence of resurgence in cases in places that have overreached a bit,” said David Rubin, director of PolicyLab.

Researchers at the University of Texas Southwestern Medical Center said last week that cases in the Dallas-Fort Worth metropolitan area could spike this summer, with a tripling of daily active cases of covid-19, the disease the novel coronavirus causes, if there is a significant easing of mitigation efforts. And PolicyLab projects that in the next month, Harris County, which includes Houston, will go from a couple hundred cases a day to more than 2,000.

The overall national picture remains ambiguous: The daily death toll from covid-19 is dropping, but increased activity and travel in a population that remains susceptible to infection means the coronavirus has new opportunities to spread.

Anthony S. Fauci, director of the National Institute for Allergy and Infectious Diseases, said Wednesday he has “no doubt” there will be new waves of cases.

“The virus is not going to disappear,” he said in an interview with The Washington Post. “It’s a highly transmissible virus. At any given time, it’s some place or another. As long as that’s the case, there’s a risk of resurgence.”

He said the country has time now to prepare for new caseloads, which could mount considerably in the fall.

A presentation prepared by the Department of Health and Human Services and the Federal Emergency Management Agency — and reviewed by The Post — suggests new waves could be steep enough in some places to overwhelm ventilator capacity.

For instance, the data indicates that only 866 ventilators are in use in Georgia, which has pursued one of the most aggressive reopening plans. But the state’s supply of 2,853 ventilators could be outstripped as soon as the end of the month by the projected number required for covid-19 patients, according to the federal modeling. States from Arizona to Colorado to Tennessee could face similar shortages, according to the projections.

An “ensemble” model that incorporates 20 leading pandemic models, developed by biostatistician Nicholas Reich of the University of Massachusetts at Amherst, shows a gradual decline in projected covid-19 deaths over the next four weeks, from about 9,000 this week to 4,000 in the second week of June. His model shows that by mid-June, the United States should expect to reach 113,000 deaths.

But Reich said Wednesday that this is a particularly difficult phase of the pandemic to capture in models because of uncertainty about how people will behave as restrictions are lifted. “There’s so much complexity and so much that could change,” Reich said.

Some communities seem to be abiding better by social distancing, Rubin said. They include Denver, Colorado Springs, Columbus, Ohio, and the Research Triangle of North Carolina, all of which look good in the new forecast.

But South Florida, which has the bulk of that state’s cases, looks worrisome in the four-week projection, Rubin said.

“That Southeast coast, they’re just starting to open up and relax. It’s a densely crowded area. There’s a lot of tinder down there,” he said.

Alabama will probably experience a steep increase in cases in nearly every county over the next month, according to the PolicyLab model.

State Health Officer Scott Harris said this week that Alabama’s numbers were “not as good as we could hope for.” The state began easing its stay-at-home order and other restrictions this month. Gov. Kay Ivey (R), who has allowed restaurants, bars, retail businesses, churches, gyms and salons to reopen, is expected to outline further steps this week.

In a news conference Wednesday, Montgomery Mayor Steven Reed said the city is facing a shortage of intensive care beds and being forced to divert patients to Birmingham. “They’re at a capacity that is not sustainable,” he said. “Our health-care system is maxed out.”

In upstate Monroe County, N.Y., home to the city of Rochester, hospitalizations of covid-19 patients have risen by about 70 percent over the past 10 days and 18 percent since reopening began on Friday. PolicyLab projects that cases in Monroe County will rise through the end of May before falling precipitously through mid-June.

Michael D. Mendoza, Monroe County’s public health commissioner, told reporters Tuesday that he believed hospitalizations have risen in large part because the county dramatically increased testing in May and is doing more testing in nursing homes and in hospitals. The number of covid-19 patients in intensive care and on ventilators has “remained relatively stable,” he said.

In Texas, there has been an outbreak of cases in El Paso and in meatpacking plants in the Panhandle. The rate of positives in coronavirus tests has gone down as the number of tests has increased, and hospitalization rates are holding steady. Gov. Greg Abbott (R) has sent “surge response teams” to places where there are spikes in infections. But the number of daily active cases is still rising in some parts of the state. Dallas and nearby Tarrant County, home to Fort Worth, each had its highest single-day death toll Tuesday.

Umair Shah, executive director of Harris County Public Health, said he has seen “slight upticks” but not “a significant increase thus far” in cases. He is concerned, however, about the state’s reopening.

“There’s mixed messages from the federal, state and local levels,” Shah said. “It’s summer, it’s 90 degrees right now outside — for people to start getting complacent and feel, ‘Oh, the virus is yesterday’s news’ . . . we have that concern.”

Lauren Ancel Meyers, who directs the Covid-19 Modeling Consortium at the University of Texas at Austin, said her group looks at cellphone data.

“It’s telling us the same thing we see when we look out our windows,” Meyers said. “We are seeing a big change in people’s mobility.”

The increased activity so far has not led to an increase in hospitalizations.

“If the changes in behavior that started in May really did accelerate transmission, we will begin to see that in the case data, the hospitalization data, the death data very soon,” she said.

One fundamental problem for scientists is that the virus does not reveal its presence readily. There is a lag in data. On average, it takes about five days for a person infected with the coronavirus to develop symptoms. That incubation period can be even longer — up to 14 days. Then there is another lag before a symptomatic person gets tested and gets the results or perhaps needs hospitalization.

“We’re looking at potentially a month or two later that we’re going to see the impact” of the reopening, said Leana Wen, the former Baltimore health commissioner. “You have not seen the impact of reopening yet. I think there’s going to be a very significant lag.”

Arelis R. Hernandez in San Antonio, Cleve R. Wootson Jr. in Tampa and William Wan in Washington contributed to this report.