Mechanistic reconstruction as complementary evidence for reported ancestral-lineage SARS-CoV-2 long-distance airborne outbreaks

Several short-duration indoor SARS-CoV-2 outbreaks before Alpha were interpreted as consistent with transmission beyond 2 m, but retrospective evidence cannot distinguish shared-air exposure from close contact. I developed a size-resolved framework to test how demanding pure long-distance airborne transmission is, using literature-derived source-to-deposition distributions, restricted to ancestral/pre-VOC conditions between January and September 2020. Four events were analysed: the Skagit and Wihr-au-Val choir rehearsals, the Hunan bus journey, and the Vaud courtroom hearing. Activity-matched particle and RNA emission rates for breathing, speaking, and singing were first compared against measurement to establish baseline source coverage and its dependence on the assumed source population. For each event, 100,000 realizations were evaluated across a scenario grid combining viral-load priors, RNA-to-TCID50 conversion scenarios, and deposition mechanisms, alongside a separate analysis retaining empirically paired RNA and FFU measurements. Compatibility required at least the reported number of susceptible occupants to deposit one or more infectious-unit equivalents (IUE), TCID50- or FFU-derived. No dose-response model was applied, as no validated function links deposited ancestral SARS-CoV-2 dose to infection. Compatibility under the RNA-TCID50 framework was consistently low under the RNA-positive viral-load priors, reaching at most 1.14% for Skagit, 0.61% for Wihr, 0.02% for Hunan, and 0.04% for Vaud. It was approximately an order of magnitude higher under the culture-positive prior, though the highest single scenario cell reached only 22.7% even under this upper-bound scenario. The matched RNA-FFU analysis was substantially lower, remaining below 0.01% in every cell, underscoring the model's high sensitivity to the choice of infectious-unit conversion basis. For the outbreaks that were reproduced as compatible, the pure long-distance airborne hypothesis required more than poor ventilation and elevated aerosol-generating activity: it also required an unusual draw from the upper tail of the viral-load distribution and, to a lesser extent, emitted liquid volume and infectious fraction. The proposed route is physically possible but demands an unusual conjunction of source and exposure conditions. This small compatible region supports caution in attributing these outbreaks to pure long-distance transmission, since unreported short-range exposure, additional infectious sources, and other unmeasured contributors remain plausible.