One of the largest events since the COVID-19 pandemic hit the U.S., last month’s Sturgis Motorcycle Rally in South Dakota was a 14-hour ride from Northwest Indiana.

Even before it drew 460,000 people, the 10-day rally got renewed attention with public health officials fearing it would become a “superspreader” event, with cases rippling across the U.S.

The average age is around 54, while attendees typically pack popular outdoor concert venues, campgrounds, tattoo joints and bars. With a culture of living freely on the road, many, according to media reports, were not wearing masks.

“I think this is one of those challenging events,” Indiana University epidemiology professor Thomas Duszynski said. “They come from all over the country. They get infected and they go back to their home.”

For public health, that presents a challenge.

An analysis of anonymous cellphone data from Camber Systems, a firm that aggregates cellphone activity for health researchers, found that 61% of all the counties in the U.S. have been visited by someone who attended Sturgis, creating a travel hub that was comparable to a major U.S. city.

As of Aug. 25, state health departments have reported 103 cases from people in South Dakota, Minnesota, Wisconsin, Nebraska, Montana, North Dakota, Wyoming and Washington. Health officials in South Dakota have said they don’t know how many people were exposed and issued public warnings of possible COVID-19 exposure at five businesses popular with bikers.

More recent reports tied 260 known cases to the rally. One Minnesotan man died of the virus since attending.

“You spread these cases out across the whole country, it doesn’t look like that big of a spike,” Duszynski said. “You need to be able to have this national perspective.”

A big issue is that fractured picture, tracking people across different states where health departments don’t have jurisdiction over each other, he said. How few bikers will contact tracers reach? Another concern is without symptoms, how many are likely to get tested who could pass it on?

Researchers from San Diego State University used cellphone tracking data and virus case numbers to model or predict a ‘worst case’ scenario. It concluded the rally could account for as many as 266,000 cases between Aug. 2 and Sept 2. With a typical coronavirus treatment case costing $46,000, health care costs for that many cases would cost $12.2 billion, they said.

In South Dakota, those figures were disputed by state health officials, who said it wasn’t peer-reviewed and didn’t match what they were seeing. Republican Gov. Kristi Noem, a Trump ally who encouraged the rally, called the findings “fiction” on Twitter. Sturgis’ economic impact was estimated at nearly $800 million in 2018.

Back in Indiana, unlike Sturgis, the Indianapolis 500 canceled crowds this year.

Roger Penske was forced to host his first 500 as owner of the iconic speedway without fans and it made the largest venue in the world still. The speedway typically draws more than 300,000 spectators on race day; Penske said there would be only 2,500 in attendance last month.

He “really wanted people there,” Duszynski said. “The science just couldn’t support that. If we don’t have some assurance we can do this safely, maybe that’s not a good idea.”

The Associated Press contributed.