Inactivation of influenza A virus in expiratory droplets and aerosol particles and the associated physicochemical drivers

Influenza is an infectious respiratory illness caused by influenza viruses. Every year, it causes up to one billion cases of disease worldwide. Despite its high disease burden, the transmission pathway of influenza remains subject to debate. There is increasing evidence that airborne transmission by exhaled aerosol particles and droplets plays a major role in the spread of the disease and its study has been declared of primary importance by the World Health Organization. Airborne transmission necessitates that influenza virus remains stable between exhalation by an infected individual and inhalation by the next host, yet its fate in exhaled aerosol particles and droplets is poorly understood. This thesis aims to identify the parameters driving the inactivation of influenza A virus (IAV) in exhaled droplets and aerosol particles, and to quantify the associated inactivation kinetics. After exhalation, particles and droplets equilibrate with the surrounding air, leading to the loss of water and the partitioning of gases, as well as the enrichment of solutes. As a result, extreme conditions of pH and salinity can be created within the particles and droplets. We therefore quantified the inactivation kinetics of IAV in simulated lung fluid (SLF) at pH 2.5 to 7, and we combined the inactivation rates with microphysical properties of SLF using a biophysical aerosol model. We found that small aerosol particles exhaled into indoor air become mildly acidic, rapidly inactivating IAV within minutes. Aerosol pH can be further decreased by enriching indoor air with non-hazardous levels of nitric acid, allowing inactivation of IAV in small aerosol particles in less than 30s. In larger droplets, acidification is slow and pH-mediated inactivation is inefficient. However, such droplets can reach supersaturated NaCl conditions within minutes. We demonstrated that IAV inactivation is driven by the increasing NaCl molality during water evaporation. By combining the experimental results with a biophysical model, we furthermore established an exponential dependence of IAV inactivation rate constants on NaCl molality in the droplets. In the presence of an organic co-solute, sucrose, the inactivating effect of NaCl was attenuated, which we attribute to two mechanisms: first, sucrose decreases the molality of NaCl during the drying phase, and second, sucrose stabilizes IAV by preventing the NaCl-promoted damage of the viral envelope.Finally, we investigated the effect of additional organic co-solutes beyond sucrose on the stability of airborne IAV. We compared IAV stability in droplets consisting of respiratory fluids and artificial saliva. We found that the extent of IAV protection increases with increasing organic:NaCl ratio. Protection mechanisms include the triggering of salt efflorescence, which reduces virus exposure to high NaCl molality, and the modulation of the drying droplet morphology. The protective effect of organics was also observed in aerosol particles, but the extent of protection differed between the two systems.Overall, this thesis contributes to a better understanding on parameters modulating airborne IAV stability. It emphasizes the role of pH and salinity and the importance of organic content in exhaled aerosol particles and droplets, shedding light on the factors affecting IAV stability and transmission in different fluid matrices. In addition, it provides insights into mitigation strategies to stop the spread of an emerging outbreak.