More than 100 coronavirus infections have been linked to the Sturgis Motorcycle Rally, an annual event that drew hundreds of thousands of enthusiasts to South Dakota as the virus’s ferocious delta variant spread misery nationwide.

Health officials in South Dakota, where the rally was held from Aug. 6 to 15, said contact tracing has connected 16 cases to the event. North Dakota identified 40 cases, while Wyoming confirmed 32, Wisconsin tallied 20 and Minnesota counted 13.

The 121 cases among the five states almost certainly represent an undercount because the rally concluded less than two weeks ago and contact tracing is challenging in connection with an event that lures attendees from around the country.

These cases are an echo of an outbreak from a year ago, during the pandemic’s first summer, when the rally was thought to have seeded hundreds of infections and contributed to a surge in the Upper Midwest.

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This summer, several counties in the Sturgis area are already reporting increases in their case rates. Although cases were trending up earlier in August, the more recent increases have been larger and faster.

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Before the rally, the Centers for Disease Control and Prevention had not classified any South Dakota county as a “hot spot.” In the weeks that followed, the agency gave that designation to Meade County, which includes Sturgis, and neighboring Pennington and Lawrence counties.

“For those of us in public health who helped document the widespread transmission of COVID-19 from last year’s Sturgis rally, we are now waiting for what will likely be next week’s outbreak reality,” said Michael T. Osterholm, an infectious-disease expert at the University of Minnesota.

Even with widespread availability of coronavirus vaccines, some health experts and other critics had questioned holding the event this year amid the rise of the highly contagious delta variant.

But unlike in 2020, many large-scale gatherings moved forward this summer, including music festivals and sporting events packed with tens of thousands of attendees. South Dakota Gov. Kristi L. Noem (R), who has stridently rejected social distancing measures aimed at limiting the virus’s spread, shrugged off concerns about the motorcycle rally.

“Bikers come here because they WANT to be here. And we love to see them!” she wrote on Facebook before the event. “There’s a risk associated with everything that we do in life. Bikers get that better than anyone.”

Nearly 526,000 vehicles passed through Sturgis during the 10 days of the rally’s 81st edition, according to a tally released by the South Dakota Department of Transportation. That was up 14% from 2020 and 5% from 2019.

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A report by researchers from the CDC and the South Dakota Department of Health found that the 2020 rally resulted in widespread transmission of the coronavirus nationwide. The study confirmed one death and at least 649 cases connected to the rally, including secondary and tertiary spread.

“To limit transmission, persons attending events should be vaccinated or wear masks and practice physical distancing if unvaccinated,” the authors wrote.

Some major gatherings this year have required vaccination or a negative coronavirus test for entry, including Chicago’s Lollapalooza, which still was connected to 203 cases. In Sturgis, there were no such rules, although the city offered coronavirus tests, masks and hand sanitizing stations. Officials also approved a temporary open-container ordinance to try to keep people outside rather than packed into bars.

Rod Woodruff, the longtime president of the Sturgis Buffalo Chip — which bills itself as the “Largest Music Festival in Motorcycling” — said he thinks the dangers of the rally amid the coronavirus pandemic have been exaggerated. He dismissed “the baloney about being the superspreader” last year as “pure poppycock.”

He couldn’t remember whether the known cases linked to last year’s rally numbered in the 60s or 600s — but either way, that didn’t seem bad to him, given the size of the event.

“It’s pretty minuscule,” he said.

Woodruff described South Dakota’s approach to coronavirus measures as “people make up their own minds about things,” but noted that some took precautions.

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The Buffalo Chip, a sprawling campground and outdoor concert site, urged people with COVID-19 symptoms to isolate immediately and asked those who tested positive for the virus to contact their medic. Woodruff said social distancing and masking backstage were enforced. He said he knows of two people who developed COVID-19 symptoms on their way to this year’s rally, tested positive and then quarantined as soon as they arrived, spending their vacation “locked up in a cabin.”

The 75-year-old said he also knows of two “anti-vax” people from the area who developed COVID-19 after joining the rally and “got pretty sick” but were not hospitalized and have improved. Like with Sturgis 2020, he didn’t view that as alarming, considering the number who attended.

Cases in Meade County had gone up fivefold in the month before the rally, from a low level of just a few per day per 100,000 residents, according to Washington Post tracking. In less than two weeks since the event ended, cases have risen to an average of about 170 cases per day per 100,000 people.

Similarly, new cases in Lawrence County, just to the west of Sturgis — previously in the low single digits — increased fourfold in the month before the event and more than fivefold since, resulting in a recent average of about 128 new infections each day per 100,000.

Pennington County, which includes Rapid City, is just south, and cases there went up by a factor of seven in the month before the event. They’ve almost quadrupled again post-rally.

Daniel Bucheli, a spokesman for the South Dakota Department of Health, said the spikes “are following a national trend being experienced in every state.”

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The impact of the rally may become clearer in coming weeks. One hospital official, Monument Health’s Shankar Kurra, told South Dakota Public Broadcasting that he expected a spike in cases to begin about 10 days after the bikers left town.

“We will see that,” Kurra said. “The question then remains: How many of those folks’ cases we see are vaccinated cases? Because the ones that are not vaccinated will end up in the hospital requiring ICU care and unfortunately may even succumb to the disease.”

Among cases linked to the rally, the severity is unclear. Four of the five states that reported Sturgis-related cases did not provide that level of detail. Minnesota officials said that five of their 13 cases were potential breakthrough infections and that none resulted in hospitalization.

Kris Ehresmann, infectious-disease director at the Minnesota Department of Health, said she anticipates more rally-related infections. The delta variant has changed the calculus on major gatherings, she said, and “is now itself a superspreader event, because it’s so much more transmissible than what we were seeing with the wild type virus.”

She noted that if the average person infected with the delta variant infects six people, as scientists believe, the state’s 13 cases could become 468 cases by their third generation.

“The much greater infectiousness and transmissibility of delta really amplifies the impact of a large event like this,” Ehresmann said. “Delta has obviously changed the landscape.”

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The Washington Post’s Jacqueline Dupree and Lena H. Sun contributed to this report.