Airborne particle size restricts SARS-CoV-2 transmission and limits infection risk in Syrian hamsters

Observational studies during COVID-19 pandemic have identified the importance of SARS-CoV-2 transmission through the air, but the size range of airborne particles that mediate SARS-CoV-2 transmission remains undefined. Using impactors to separate airborne particles at specific cut-off sizes, we showed that SARS-CoV-2 is transmitted efficiently in Syrian hamsters via airborne particles ranging from 1.5 to 15 um. Infection risk was correlated with the size distribution of virus-laden particles, which did not follow the size distribution of total airborne particles sampled from naive hamsters. Reducing the size of airborne particles resulted in a narrow transmission bottleneck. Our results suggest that airborne particle size may serve as a physical constraint limiting the number of infectious virions per particle, affecting the risk of infection and possibly the size of the viral transmission bottleneck.