When fluid mechanics meets virology: revisiting smallpox – a modeling framework for its airborne transmission

Combining virologic data with inhaled particulate dynamics inside respiratory cavities can model the infection onset process for pathogens that are either airborne or are anisotropic with predominantly aerial transmission routes. Here we apply this approach to answer the following questions on smallpox infection: (a) which droplet/aerosol sizes are mainly responsible for ferrying the virions to the infective zones at the oropharynx and in the lower airway; (b) what is its infectious dose, i.e. how many virions can trigger the infection. We have linked LES-based computational tracking of inhaled transport in 3 CT-based anatomic airway geometries – with the virion concentration in respiratory ejecta of a diseased carrier and the droplet/aerosol size distribution that an exposed subject might inhale. The tested droplet/aerosol sizes ranged over 0.1–35 microns and entered the airway through nostrils and mouth for steady-to-moderate breathing rates of 15, 30, 55, and 85 L/min. Viral transmission to the lower airway peaked for droplet diameters 0.1–14 microns; the corresponding range for oropharynx was 7–35 microns. Using literature data on confirmed exposure durations for transmission, this study approximates the smallpox infectious dose at 200–2000, which agrees with known estimates.