  In temperate climates, infection rates of enveloped viruses peak during the
winter. While these seasonal trends are established in influenza and human
coronaviruses, the mechanisms driving the variation remain poorly understood
and thus difficult to extend to similar viruses like SARS-CoV-2. In this study,
we use the Quadrature-based model of Respiratory Aerosol and Droplets (QuaRAD)
to explore the sensitivity of airborne transmission to the seasonal variation
in indoor relative humidity across the wide range of relevant conditions, using
SARS-CoV-2 as an example. Relative humidity impacts the evaporation rate and
equilibrium size of airborne particles, which in turn may impact particle
removal rates and virion viability. Across a large ensemble of scenarios, we
found that the dry indoor conditions typical of the winter season lead to
slower inactivation than in the more humid summer season; in poorly ventilated
spaces, this reduction in inactivation rates increases the concentration of
active virions, but this effect was important when the susceptible person was
farther than 2 m downwind of the infectious person. On the other hand, changes
in particle settling velocity with relative humidity did not significantly
affect the removal or travel distance of virus-laden scenarios.
