Comparison of the COVID-19 infection risks by close contact and aerosol transmission

A comprehensive understanding of the transmission routes of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is of great importance for the effective control of the spread of Corona Virus Disease 2019 (COVID-19). Human-to-human transmission by close contact where large respiratory droplets play a significant role has been established as the main transmission route. At the same time the transmission by small aerosol is getting increasing attention. There is no distinct boundary between droplets and aerosol in nature so it is natural to investigate the infection risk due to aerosol. Here, we utilized a newly developed dose-response relation, combined with a box model for the exposure estimation, to quantitatively evaluate the infection risk of SARS-CoV-2 through aerosol transmission and compared with the risk due to close contact. The results indicated that the median infection risk via aerosol transmission was about 3.7x10-5 (95% confidence interval: 3.5x10-6 to 4.4x10-4) for one hour of exposure in a room with the size of 10 m (width)x10 m (length)x3 m (height) with one infected individual in it. The risk was more than three orders of magnitude lower than the risk at short distance, about 12.8% within 1 m, based on a meta-analysis. A simple exponential regression model Risk=10-0.90xD+0.10 (D<=5 m) could be utilized to characterize the magnitude of infection risk in the considered scenario based on the distance D from the infected individual. With prolonged exposure duration and large exposed population, the infection caused by aerosol transmission could be considerable, thus it is necessary to be cautious for the potential aerosol transmission risk in such situations.