After Memorial Day, as the United States began to reopen, coronavirus cases began to rise. The number of hospitalizations had risen in at least a dozen states some two weeks after the holiday weekend.

For the Fourth of July weekend, a new analysis of cell phone data suggests even more people hit the road among 10 coronavirus hotspots, despite warnings from health experts.

Mobility, experts say, is one driver of transmission of the novel coronavirus.

The analysis comes from data shared with CNN by Cuebiq, one of the private companies that the US Centers for Disease Control and Prevention uses to track general movement in the United States. Cuebiq gets its data when people download apps on their phones and opt into anonymous location data tracking. The company’s full data set includes 15 million phones nationwide.

For the Fourth of July holiday weekend, the data show that the numbers of people traveling were generally higher overall than Memorial Day weekend. The July 4 weekend was generally recognized as Friday, Saturday and Sunday, while Memorial Day weekend in May was Saturday, Sunday and Monday.

Cuebiq focused on the number of visitors to and from 10 metro area hotspots from a week prior to the July 4 weekend.

The metropolitan areas had been ranked by the percent positive Covid-19 tests the week prior in a presentation by the White House Coronavirus Task Force. The analysis included the Houston; Austin, Texas; Phoenix; Dallas; San Antonio; Orlando; Tampa, Florida; Charleston, South Carolina; Miami; and Atlanta areas.

Local leaders were concerned that gatherings over the July 4 holiday could spread the virus. States put an end to some celebrations. California Gov. Gavin Newsom urged cities struggling with the virus to cancel fireworks. Beaches and parks in Miami were closed over the holiday. Some beaches in South Carolina canceled their fireworks shows. In Texas, tubing was canceled on some rivers. Some places celebrated with livestreamed events.

Still, the draw of a three-day summer weekend with long-awaited get-togethers and newly reopened attractions worried health experts.

“It’s set up a perfect storm: the combination of travel, the combination of reopening — perhaps in some cases, too early — and the combination of people not necessarily following some of these preventive guidelines,” Dr. Joshua Barocas, an infectious disease physician at Boston Medical Center, said before the holiday.

Metro areas with the most movement

Despite the warnings, nearly all of the metro areas Cuebiq analyzed saw an increase in the proportion of visitors over the Fourth of July weekend, when compared to either Memorial Day weekend or the two-week prior average. The sole exception was the Phoenix area, which saw a slightly higher percentage of visitors during Memorial Day weekend, but whose locals traveled elsewhere in greater numbers over Fourth of July weekend.

Among them, two hotspot cities saw the highest proportions of vacationers: Orlando saw the largest increase in visitors compared to the weeks leading up to the holiday weekend and Charleston saw the highest percentage of visitors among the 10 metro areas. In both areas, roughly 1 in 5 devices was determined to be a visitor, according to Cuebiq’s analysis. Visits have been steadily increasing in Charleston through June, the company’s data showed.

Travelers tended to visit cities in their own state or region, but some traveled further. About 3.7% of visitors to the Miami area came from New York, and another 3.9% came from the Atlanta area. Of the people who visited Phoenix, 16.3% came from just three metro areas in Southern California — those including Los Angeles, Riverside and San Diego. Others came from areas around Chicago and Dallas — about 2% of Phoenix visitors in each case.

Of these 10 areas, Atlanta area residents were the most likely to travel. About 20% of the resident devices the company tracked had left the state and traveled to another city during the July 4 weekend. Many went to Florida.

‘When we see social mobility going up … we see a rise in cases’

As more states open up, people have generally been on the move more, even as the number of Covid-19 cases continues to rise in the United States. CDC’s aggregate mobility data, which includes Cuebiq, shows steadily increasing mobility trends since mid-April, especially to retail and recreation locations.

Mobility doesn’t equal infection; maintaining physical distance and wearing masks can reduce the risk of the coronavirus spreading. It could be weeks before states notice an increase in cases linked to holiday getaways, if they happen. But with travel, there are a lot more opportunities for people to come into contact with someone who can make them sick.

Health experts predict that there will be an uptick in cases the more people are mobile. Dr. Cassandra Salgado, the infectious diseases medical director at the Medical University of South Carolina, has been watching the crowds grow in Charleston. To prepare the medical system to be ready for additional patients, her group has been following a number of social mobility models during the pandemic, including Cuebiq’s, and the models line up with the uptick in cases.

“When we see social mobility going up, in seven to ten days later, we see a rise in cases, and then we see a bigger rise, as we are starting from a higher starting point. These models are really useful to help us prepare,” Salgado said.

She said Charleston is a beautiful tourist destination and she understands why so many people would want to visit. Plus, it is “pretty much wide open,” as far as restaurants and bars. People are asked to wear a mask when they can’t social distance.

“Some people are being very responsible, but some aren’t unfortunately,” she said. “I have always sort of said, human beings, if things are open for them, they will go do them.”

Salgado said there have been moments in the pandemic when she has been proud of people for staying home, but she has also felt some disappointment and frustration.

“I worry mostly about the vulnerable population who don’t have a choice and are somewhat victims of what other people decide to do,” Salgado said.

She said she can see why sometimes people will see that more is reopening and start to think it is safe to go out or to travel, especially in the summer vacation season. But she said they definitely have seen mini outbreaks tied to travel.

“We sure will keep a close eye on it every day,” Salgado said. Typically, hospitalizations won’t spike for another few weeks.

“It’s still quite early, but we’ll prepare and be ready for it.”