Loss of Infectivity of Influenza Virus and SARS-CoV-2 during Aerosol Sampling

Our understanding of transmission of influenza virus and other respiratory viruses is limited by the difficulty of detecting infectious viruses in aerosol particles. Most aerosol sampling methods are believed to contribute to virus inactivation, but the magnitude of this sampling artifact is unknown. To investigate this question, we aerosolized influenza A virus (IAV) and SARS-CoV-2 suspended in human saliva into a small chamber (3.7 L). Aerosols settled for 10 min onto either cells or a thin layer of liquid medium that was immediately transferred to cells for plaque assay. Aerosols that deposited directly onto cells led to the formation of 100× more plaque forming units (PFU) compared to aerosols that deposited first into liquid medium. Further experiments ruled out uneven aerosol distribution in the chamber or inefficient virus recovery as causes of this discrepancy. These findings indicate that aerosolized IAV and SARS-CoV-2 lost infectivity by approximately 2 log₁₀ PFU within ∼10 min unless they attached to cells quickly. As natural infection via inhalation occurs by direct deposition of the virus onto cells, we hypothesize that sampling directly onto cells more accurately reflects the potential for exposure to lead to infection.