The rapid natural viral inactivation in an evaporating droplet

Abstract

The role of expired droplets in respiratory disease transmission has become one of the most heated topics during the COVID-19 pandemic, yet the relations between the infectivity decay in droplets and its contributing factors were still unclear. Among these factors, humidity and temperature were relatively well studied with animal models, droplet experiments, and droplet plume simulations. Nonetheless, the experimental data failed to reveal the cause of infectivity decay of viruses suspended in a droplet. There was also a lack of knowledge on the timing of the inactivation process. As a result of these, there are many unresolved questions: For example, what is the droplet infection transmission effectiveness during a specific human face-to-face activity? How do we apply the findings of viral viability decay to everyday life? Does the initial temperature of the infectious droplet plume contribute to the transmission risk apart from the droplet trajectory? Without this information, scholars relied on droplet transportation models to estimate the risk while neglecting the infectivity loss due to environmental exposure. One major technical limitation was the ineffectiveness of the current experimental setups in measuring viral viability in rapidly evaporating infectious droplets. The latest experimental data showed approximately 54-μm droplets can lose over 40% of infectivity in the first five seconds at 40% relative humidity. This would happen faster on expired droplets with typically <10-μm diameters. The conventional rotating drum setup was normally used for capturing droplets after hours of environmental exposure, and the advanced levitation method struggled to capture droplets at <5 seconds. By capturing the initial rapid infectivity loss in droplets, a comprehensive evaluation can be done to unravel the underlying mechanisms, which then facilitates future studies on transmission risks under convoluted settings. Here, a droplet capturing shaft was proposed based on the free-fall technique and mono-sized droplet generation with electromechanical modifications to prevent droplet coalescence. This approach has successfully provided the very first measurement on viability decay from ~100 milliseconds to >8 seconds of microscale droplets. A mechanistic multi-species drying droplet model, namely the continuous species transport with population balance (CST-PB), was introduced to estimate the hard-to-measure droplet internal composition during its rapid evaporation and crystallization processes. Enveloped virus surrogate was suspended in ~61-μm salt-water droplets, which were captured during the initial evaporation process. Viability was measured with plaque assay and RT-qPCR, and the results were analyzed alongside the estimated physicochemical droplet properties. The analysis revealed a correlation between the infectivity loss and osmotic shock due to rapid evaporation. In fact, viability loss from osmotic shock in, for instance, sample dilution has been reported. This provided new insights into previous findings, such as the zones of infectivity decay and the connection between the virus’ host cell and its resilience to environmental conditions. The drying droplet model also demonstrated a method to predict the infectivity decay timing in other droplet compositions, droplet sizes, and environmental conditions. In the future, this approach may serve as a framework to extend the measurement to intermediate conditions and safely examine infectivity loss without aerosolization.