Modelling airborne transmission of SARS-CoV-2 using CARA: Risk assessment for enclosed spaces

The COVID-19 pandemic has highlighted the need for a proper risk assessment of respiratory pathogens in indoor settings. This paper documents the COVID Airborne Risk Assessment (CARA) methodology, to assess the potential ex- posure of airborne SARS-CoV-2 viruses, with an emphasis on virological and immunological factors in the quantification of the risk. The model results from a multidisciplinary approach linking physical, mechanical and biological do- mains, enabling decision makers or facility managers to assess their indoor set- ting. The model was benchmarked against clinical data, as well as two real-life outbreaks, showing good agreement. A probability of infection is computed in several everyday-life settings and with various mitigation measures. The import- ance of super-emitters in airborne transmission is confirmed: 20% of infected hosts can emit approximately two orders of magnitude more viral-containing particles. The use of masks provide a 5-fold reduction in viral emissions. Nat- ural ventilation strategies are very effective to decrease the concentration of virions, although periodic venting strategies are not ideal in certain settings. Although vaccination is an effective measure against hospitalization, their ef- fectiveness against transmission is not optimal, hence non-pharmaceutical inter- ventions (ventilation, masks) should be actively supported. We also propose a critical threshold to define an acceptable risk level.