Presymptomatic shedding is heterogeneous. A) Distribution of presymptomatic shedding across all individuals with ILI symptoms (N = 105). B) Association between presymptomatic shedding and age group. C) Association between presymptomatic shedding and vaccination status. Presymptomatic shedding was estimated as the percentage of total virus shedding that occurred prior to ILI symptom onset. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.