Quantifying multi-virion aerosols in respiratory infection transmission

Infectious aerosols are expired in various sizes, and the number of contained virions is expected to significantly vary among expired aerosols. To evaluate the effect of the number of contained virions on infection risk, multi-virion aerosols in expired air should be first quantified. In this study, we categorised expired aerosols into traditional aerosol size bins and aerosols containing different number of virions. We quantified the number of virions in aerosols of each size bin and estimated the size distribution of aerosols which contain a certain number of virions when the viral load is known. This analysis helped us to determine the relationship between multi-size aerosols and multi-virion aerosols. Our findings indicate that for long-range airborne transmission, the airborne transmission droplets derived from four COVID-19 outbreak venues contain at most one infectious virion, supporting the one-virion assumption for the considered viral load ranges. However, for short-range airborne transmission, the impact of multi-virion aerosols on infection risk must be considered. Our quantification of multi-virion aerosols for the first time lays the groundwork for incorporating the multi-virion assumption into infection risk assessments.