WashU Expert: Without requiring vaccines, filled stadiums are unsafe Circa April 2021: Indianapolis Motor Speedway, Gate Two Entrance. IMS is preparing for the Indy 500 and Brickyard 400 in the age of social distancing. (Photo: Shutterstock) June 1, 2021 SHARE Some 135,000 fans gathered in the Indianapolis Motor Speedway for the Indy 500 over the 2021 Memorial Day Weekend. A Washington University in St. Louis mathematician, who helped write a scientific formula and a paper regarding the risk analysis of fans attending sporting events, has a simple, evidence-driven answer to two questions about that race crowd: How many were vaccinated? How many were safe from COVID-19 transmission? Not enough.