The research, led by B. Douglas Bernheim, chair of economics at Stanford University, analyzed data following 18 Trump rallies held between June 20 and Sept. 22, three of which were indoors. Infectious disease experts have long suspected that the president's rallies ahead of the Nov. 3 election might be so-called superspreader events. In recent months, Trump has held several dozen rallies in states such as Pennsylvania, Minnesota and Wisconsin, where coronavirus infection rates were already on the rise.