Review of Infective Dose, Routes of Transmission, and Outcome of COVID-19 Caused by the SARS-CoV-2 Virus: Comparison with Other Respiratory Viruses

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is pandemic. Prevention and control strategies require an improved understanding of SARS-CoV-2 dynamics. We rapidly reviewed the literature on SARS-CoV-2 viral dynamics with a focus on infective dose. We sought comparisons of SARS-CoV-2 with other respiratory viruses such as SARS-CoV-1 and MERS. We examined laboratory animal, and human studies. The literature on infective dose, transmission, and routes of exposure was limited, especially in humans, and varying endpoints were used for measurement of infection. The evidence, albeit limited, indicated that the minimum infective dose of COVID-19 in humans, is higher than 100 particles, slightly higher than the hundreds of particles estimated for SARS-CoV-1. Despite variability in animal studies, there was some evidence that increased dose at exposure correlated with higher viral load clinically, and severer symptoms. Higher viral load measures did not reflect COVID-19 severity. Aerosol transmission seemed to raise the risk of more severe respiratory tract complications. An accurate quantitative estimate of the infective dose of SARS-CoV-2 in humans is not currently feasible and needs further research. Further work is also required on the relationship between routes of transmission, infective dose, and outcomes.