Environmental Stability of Enveloped Viruses is Impacted by the Initial Volume and Evaporation Kinetics of Droplets

Abstract

Efficient spread of respiratory viruses requires the virus to maintain infectivity in the environment. Environmental stability of viruses can be influenced by many factors, including temperature and humidity. Our study measured the impact of initial droplet volume (50, 5, and 1 µL) and relative humidity (RH: 40%, 65%, and 85%) on the stability of influenza A virus, bacteriophage, Phi6, a common surrogate for enveloped viruses, and SARS-CoV-2 under a limited set of conditions. Our data suggest that the drying time required for the droplets to reach quasi-equilibrium (i.e. a plateau in mass) varied with RH and initial droplet volume. The macroscale physical characteristics of the droplets at quasi-equilibrium varied with RH but not with initial droplet volume. We observed more rapid virus decay when the droplets were still wet and undergoing evaporation, and slower decay after the droplets had dried. Initial droplet volume had a major effect on virus viability over the first few hours; whereby the decay rate of influenza virus was faster in smaller droplets. In general, influenza virus and SARS-CoV-2 decayed similarly. Overall, this study suggests that virus decay in media is closely correlated with the extent of droplet evaporation, which is controlled by RH. Taken together, these data suggest that decay of different viruses is more similar at higher RH and in smaller droplets and is distinct at lower RH and in larger droplets. Importantly, accurate assessment of transmission risk requires use of physiologically relevant droplet volumes and careful consideration of the use of surrogates.

Stability of influenza A virus in droplets and aerosols is heightened by the presence of commensal respiratory bacteria

“…After this sharp decay, remaining infectious viral titers were stabilised, giving an inverted sigmoidal decay curve over the entire 60-minute time-course. This sigmoidal decay for IAV in droplets has been observed before (18,19), with the sharpest decline in infectious titers occurring just prior to droplet efflorescence, when the salt molality of the droplets is maximal.…”

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

“…Similarly, Niazi et al 31,32 showed that rapid evaporation and hence rapid efflorescence stabilizes IAV and human rhinovirus-16 in airborne aerosol particles, suggesting that increased stability is due to a shorter exposure to high salinity. An important role of evaporation dynamics on IAV inactivation in droplets was also proposed by French et al, 23 who observed faster inactivation during the initial evaporation step, while the particle is still liquid, and a slower decay after efflorescence. Likewise, a near-instantaneous 50% loss in SARS-CoV-2 infectivity was observed to be associated with efflorescence, followed by a slower inactivation rate.…”

“…In typical indoor environments in temperate climates, RH can be as low as 30% during winter, whereas it is around 50% in summer. 8 The RH, [9][10][11][12][13][14][15][16][17][18][19] the temperature, [20][21][22] and the initial size and composition of the expiratory particle [23][24][25] dictate the droplet drying rate and hence the concentration of solutes in the droplets. If the RH is below the efflorescence relative humidity (ERH) of the salts, the salts will effloresce.…”

Stability of Aerosolized SARS-CoV-2 on Masks and Transfer to Skin

“…8−15 SARS-CoV-2 has been shown to decay faster in smaller droplets compared to larger ones (1 vs 50 μL) under some conditions. 18 Therefore, it is essential to examine the survival of SARS-CoV-2 deposited on masks in aerosolized form.…”