The Dynamics of SARS-CoV-2 Infectivity with Changes in Aerosol Microenvironment

Abstract

Understanding the factors that influence the airborne survival of viruses such as SARSCoV2 in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission. We present measurements of the stability of SARSCoV2 in aerosol droplets (5 to 10 micrometres equilibrated radius) over timescales spanning from 5 seconds to 20 minutes using a novel instrument to probe survival in a small population of droplets (typically 5-10) containing ~1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to 10 % of the starting value was observable for SARS-CoV-2 over 20 minutes, with a large proportion of the loss occurring within the first 5 minutes after aerosolisation. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallise at RHs below 50% leading to a near instant loss of infectivity in 50 to 60% of the virus. However, at 90% RH the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 minutes, beyond which it decays to only 10% remaining infectious after 10 minutes. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilisation of CO2 from bicarbonate buffer within the droplet. Three different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.

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

“…The biphasic virus decay that is readily observed in droplets likely occurs in aerosols, too, as shown in published work and a preprint. 2,9,24,25 While a droplet/aerosol is wet and evaporation is still occurring, the virus is subject to a faster decay rate than after the droplet/aerosol reaches a solid or semi-solid state at quasi-equilibrium, 5 as we also observed for all droplet sizes tested. At the point of efflorescence (the crystallization of salts as water evaporates), if it occurs, there appears to be a rapid loss in infectivity.…”

Modelling the impacts of public health interventions and weather on SARS-CoV-2 Omicron outbreak in Hong Kong

“…On the other hand, a very weak association between relative humidity and Re was found. The relationship between relative humidity and the transmissibility has been studied but with different conclusions [12][13][14][26].…”

Experimental measurement of respiratory particles dispersed by wind instruments and analysis of the associated risk of infection transmission

“…6 is given by the source term (which is the emission of pathogen copies into the room by the infectious) and four sink term: (i) the removal of pathogens via inhalation by the infectious, (ii) the removal of pathogens via inhalation by the susceptible (this yields the absorbed dose per time for the susceptible, which determines the infection risk), (iii) the removal of pathogens due to ventilation of the room or usage of an air purifier with β v in the unit of air changes per second, and (iv) the pathogen inactivation γ, which is approximately 0.64 h −1 as described in van Doremalen et al (2020). A recent study by Oswin et al (2022) shows that pathogen inactivation might depend on relative humidity and ambient CO 2 concentration and could actually be faster compared to the data in van Doremalen et al ( 2020). As we are interested in the upper bound on exposure to infectious particles, we use the more conservative inactivation rate by van Doremalen et al (2020).…”

“…Another aspect that introduces uncertainty to the model is that the dependence of ρ p on particle origin site and time during the course of the disease is not known. There is also a recent discussion about the inactivation rate for particles containing SARS-CoV-2, which could depend on several parameters of the room air and could be higher (Oswin et al, 2022). On the other hand, simpler models like the ones used in Miller et al (2020), Stockman et al (2021) and Firle et al (2021) have the same issues while they also do not take the size to volume ratio of the particles into account.…”