  Airborne transmission is now believed to be the primary way that COVID-19
spreads. We study the airborne transmission risk associated with holding
in-person classes on university campuses. We utilize a model for airborne
transmission risk in an enclosed room that considers the air change rate for
the room, mask efficiency, initial infection probability of the occupants, and
also the activity level of the occupants. We introduce, and use for our
evaluations, a metric $R_0^{eff}$ that represents the ratio of new infections
that occur over a week due to classroom interactions to the number of infected
individuals at the beginning of the week. This can be seen as a surrogate for
the well-known $R_0$ reproductive number metric, but limited in scope to
classroom interactions and calculated on a weekly basis. The simulations take
into account the possibility of repeated in-classroom interactions between
students throughout the week. We presented model predictions were generated
using Fall 2019 and Fall 2020 course registration data at a large US
university, allowing us to evaluate the difference in transmission risk between
in-person and hybrid programs. We quantify the impact of parameters such as
reduced occupancy levels and mask efficacy. Our simulations indicate that
universal mask usage results in an approximately $3.6\times$ reduction in new
infections through classroom interactions. Moving 90% of the classes online
leads to about $18\times$ reduction in new cases. Reducing class occupancy to
20%, by having hybrid classes, results in an approximately $2.15-2.3\times$
further reduction in new infections.
