More than 330 coronavirus cases and one death were directly linked to the rally as of mid-September, according to a Washington Post survey of health departments in 23 states that provided information.
That number likely represents just the tip of the iceberg, according to experts.
Contact tracing often doesn’t capture the source of infection, and asymptomatic spread goes unnoticed.
Within weeks of the Sturgis rally, South Dakota, North Dakota, Wyoming, Minnesota, and Montana were leading the surge in new coronavirus cases.
The Dakotas saw case numbers and rates of hospitalisation rise enormously throughout September, and into October.
It may never be confirmed how many cases stemmed from the rally, which attracted approximately 460,000 people over 10 days, according to one estimate.
Unlike most other states, South Dakota imposed no restrictions on the number of people allowed to congregate, making the Sturgis rally the largest gathering of people in the US, possibly in the world, during the pandemic.
Tens of thousands of attendees then travelled home across the country after being packed into bars, restaurants, and concert venues — few wearing masks.
Cellphone data was used to track where attendees originated from and returned to in analysis by the Centre for New Data.
South Dakota saw its seven-day rolling average number of cases rise from 84 on 6 August to 214 of 27 August. By 1 October that figure had reached 434.
Governor Kristi Noem echoed Donald Trump by saying that the increase in cases was as a result of an increase in testing. She did not explain how this accounted for the increase in hospitalisations across the state.
As of 17 October, the state has recorded 33,269 confirmed cases of Covid-19 and 323 deaths.
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