Covid transmission in crowds: Can outdoor situations be ranked by the risks that they present?
In many cities in France, it is currently compulsory to wear a mask in public places (both outdoor and in shops and public infrastructure), a measure which was imposed notably in order to mitigate the risks of viral transmissions in crowds. But in what (outdoor) situations is this risk highest, if masks are not worn by everybody?
Obviously, given the main mode of transmission of the disease (through droplets), this transmission risk will depend on the pedestrians' activity (breathing, talking, jogging, ...), on their interpersonal spacings, but also on their relative orientations (you are less likely to be infected if you turn your back to a diseased person than if you face them).
This plain observation led us to measure the spacings and orientations between pedestrians in different outdoor scenarios, within the frame of ANR project SeparationsPietons. The collated measurements were then integrated into different ad hoc viral transmission models to estimate the number of infections that would result from a contagious pedestrian walking in the crowd. Quite interestingly, even though the absolute risks considerably vary with the details of the models, we found robust features in the ranking of the scenarios for all models.
We have recently sent a summary of our preliminary results to deciders and we are now working to extend the range of situations to be explored.