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).