  Airborne viruses, such as influenza, tuberculosis, and SARS-CoV-2, are
transmitted through virus-laden particles expelled when an infectious person
sneezes, coughs, talks, or breathes. These virus-laden particles are more
highly concentrated in the expiratory jet of an infectious person than in a
well-mixed room, but this near-field enhancement in virion exposure has not
been well quantified. Transmission of airborne viruses depends on factors that
are inherently variable and, in many cases, poorly constrained, and quantifying
this uncertainty requires large ensembles of model simulations that span the
variability in input parameters. However, models that are well-suited to
simulate the near-field evolution of respiratory particles are also
computationally expensive, which limits the exploration of parametric
uncertainty. In order to perform many simulations that span the wide
variability in factors governing transmission, we developed the
Quadrature-based model of Respiratory Aerosol and Droplets (QuaRAD). QuaRAD is
an efficient framework for simulating the evolution of virus-laden particles
after they are expelled from an infectious person, their deposition to the
nasal cavity of a susceptible person, and the subsequent risk of initial
infection. We simulated 10,000 scenarios to quantify the risk of initial
infection by a particular virus, SARS-CoV-2. The predicted risk of infection
was highly variable among scenarios and, in each scenario, was strongly
enhanced near the infectious individual. In more than 50% of scenarios, the
physical distancing needed to avoid near-field enhancements in airborne
transmission was beyond the recommended safe distance of two meters (six feet)
if the infectious person is not wearing a mask, though this distance defining
the near-field extent was also highly variable among scenarios.
