Low Transmission of Coronavirus via Aerosols during Outdoor Running Races and Athletic Events

Introduction: Outdoor contacts were reported to rarely result in transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Little is known, however, about the risk during popular outdoor running events. This study assessed the transfer of aerosols from infected runners to other race participants. Methods: In the experimental part of the study, a group of dummies was pulled at different speeds over an athletics track and field circuit. Fine aerosols were produced with a fog machine, large aerosols with a pesticide sprayer releasing food colorant, with the size matching the two size modes of human expiratory aerosols. The experimentally determined transfer rates fed a Monte Carlo simulation of different race distances, starting sequences and block sizes. Runners were modeled using start and end times of SwissCityMarathon ??? Lucerne participants and a previously published distribution of virus emission strengths. The race distance was divided into 10-meter segments in which the transfer from the sources to collocated runners was calculated. Results: The experiments showed that fog and spray transfer decreased with increasing distance from the source. Increased speed was associated with decreased fog but increased spray transfer. The simulations suggest that more runners received small amounts of virus by fog-transfer. However, critical virus-transfers defined as more than 100 virus copies happened mostly by spray. The estimated rate of people getting a potentially infectious dose was in most races well below the simulated prevalence rate of virus-emitting runners, mostly about five-fold smaller. Changing from block starts to individual starts further reduced the estimated transfer. Only an artificial group running 30 km in close distance at high speed brought the rate above parity. Discussion: These findings suggest that outdoor running events are associated with a low risk for virus infection as long as runners are not trailing each other over very long distances.