Salt supersaturation as accelerator of influenza A virus inactivation in 1-μl droplets

Abstract

Influenza A virus (IAV) spreads through airborne particles ranging from sub-micrometer to millimeter-sized droplets. The stability of airborne IAV remains difficult to estimate and depends, among others, on the respiratory matrix composition and the size-dependent drying kinetics. Here, we combine experiments on deposited millimeter-sized saline droplets with a biophysical aerosol model to quantify the effect of NaCl on IAV stability, in the presence and absence of sucrose as organic co-solute. We demonstrate that IAV inactivation in dilute saline droplets after exposure to air at various relative humidities is driven by the increasing NaCl molality during water evaporation, rather than by efflorescence, i.e. the precipitation of salt crystals. For pure aqueous NaCl droplets, we find the inactivation rate constant to depend exponentially on NaCl molality, which enables us to simulate inactivation under both efflorescing and deliquescing conditions. We provide evidence for virion integrity loss as the primary inactivation mechanism. Addition of sucrose attenuates IAV inactivation, which we attribute to two mechanisms: first, sucrose decreases the NaCl molality during the drying phase, and second, at equal NaCl molality, sucrose shields IAV from the inactivating effects of NaCl. These experiments help advance our understanding of IAV stability in expiratory particles and the associated transmission risks.

Dependence of aerosol-borne influenza A virus infectivity on relative humidity and aerosol composition

“…Bulk experiments, with a w equal or very close to 1, are consistent with large droplet experiments at 100% RH given the very dilute nature of the solution and the large salt-to-virus mass ratio found in both types of experiments. The t 99 value reported by Schaub et al (2023) in bulk reaches more than 10 5 s, higher than any other value retrieved at lower RH. To compare to the results of the LAPI BREATH aerosol experiments, where it is technically impossible to achieve 100% RH conditions in the chamber, we performed additional bulk experiments corresponding to 90% and 73% RH, which yield t 99 a factor of 6 and 13 lower than at 100% RH, respectively, but still about an order of magnitude slower than the aerosol in the chamber.…”

“…Figure 3 compares t 99 of our aerosol experiments with the results of studies using microliter droplets (green symbols) by Yang et al (2012) and Schaub et al (2023). These studies also investigated the infectivity of IAV in aqueous solutions, namely PBS and aqueous NaCl solution, respectively.…”

“…Similar to other organic/inorganic mixtures (Song et al, 2013), we consider it very likely that aerosol particles originating from a PBS+sucrose medium in the LAPI BREATH only effloresced at very low RH or did not effloresce at all, regardless of RH. However, we cannot exclude the formation of microcrystals, as described by Schaub et al (2023), which do not grow because of the high viscosity and low diffusivity.…”

“…The concentrations of NaCl and sucrose in the inoculated medium examined were 8 g/L and 64 g/L respectively, corresponding to an NaCl:sucrose dry mass ratio of 1:8. This NaCl:sucrose ratio is sufficient to suppress efflorescence at 55% RH (Schaub et al, 2023).…”

“…In Figure 3, we compared microliter droplet experiment results from Yang et al (2012) and Schaub et al (2023) with our results from aerosol particle experiments, all using saline media. The fast equilibration of aerosol particles and microliter droplets to ambient RH above ∼50 % should, in principle, make the results of both types of experiments comparable.…”