Case selection and propensity adjustment
Naturally infected cases were sampled from a symptomatic population, selected on the basis of positive QuickVue® rapid test or febrile illness >37.8C (measured at University Health Center where some recruitment took place, or upon presentation to research clinic) plus cough or sore throat, and included in analysis based on a single qRT-PCR-positive nasopharyngeal swab on the day of enrollment. Experimentally infected cases were selected on qRT-PCR detection of virus from nasopharyngeal swabs on at least two of six follow-up days, or on one day plus serological evidence of infection. Differences in study design were expected to introduce imbalance in symptom severity distribution between groups (SI Appendix 2). If symptoms are associated with aerosol shedding in an unselected population, then this would be an important variable to control for with the goal of assessing differences in shedding between the groups. To minimise the effect of this bias, we attempted to balance covariate distributions between populations with propensity score models (SI Appendix 3).