A massive motorcycle rally held in the US state of South Dakota last month may have been responsible for over 260,000 new coronavirus cases, which would represent the biggest Covid-19 spreading event documented in the US to date.The study was published by the Institute of Labour Economics and conducted by economists at the University of San Diego who studied the Sturgis Motorcycle Rally, which drew an estimated 460,000 people between August 7 and 16. The paper has not yet been peer-reviewed.The…