STURGIS, S.D. (NewsNation) — The hundreds of thousands of bikers who attended the Sturgis Motorcycle Rally may have departed western South Dakota, but public health departments in multiple states are trying to measure how much and how quickly the coronavirus spread at gatherings and businesses before people traveled home to nearly every state in the country.

From the city of Sturgis, which is conducting mass testing for its roughly 7,000 residents, to health departments in at least six states, health officials are trying to track outbreaks from the 10-day rally which ended on Aug. 16. 

Health departments in four states, including South Dakota, Minnesota, Nebraska and Wyoming, have reported a total of 81 cases among people who attended the rally. South Dakota health officials said Monday they had received reports of infections from residents of two other states — North Dakota and Washington.

The state’s Department of Health also issued public warnings of possible COVID-19 exposure at five businesses popular with bikers, saying it didn’t know how many people could have been exposed.

Although the city arranged to have 1,300 tests available, about 850 people have signed up for tests so far, according to Danial Ainslie, the city manager.

The local school district delayed the start of in-person classes this year in hopes it would give health officials time to contain an outbreak. The city also made coronavirus tests available for school staff, in addition to requiring city employees to get tested.

One city employee being tested told NewsNation affiliate KELOLAND that he appreciates the testing.

“I think everybody in town should be tested, myself, just so we know how bad the virus is in town. I want to keep my kids safe. It’s nice to know if we do have the virus or not,” Daniel Tammi said.

Kris Ehresmann, infectious disease director at the Minnesota Department of Health, on Friday advised people to quarantine for two weeks if they attended the rally.

“We’re expecting that we’re going to see many more cases associated with Sturgis,” Ehresmann said.

An analysis of anonymous cell phone data from Camber Systems, a firm that aggregates cell phone 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, the Associated Press reported.

The use of cell phone data for this purpose is sparking a debate over privacy and ethics, according to a study to be published in The Lancet.