Compared with the average transmissibility of human influenza A virus infection, much less attention has been paid to its potential variability. Considering viral shedding as a proxy for infectiousness, we explored the heterogeneity of infectiousness among medically-attended cases with seasonal influenza A virus infection. The analysis revealed that viral shedding is more heterogeneous in children than in adults. The top 20% most infectious children and adults were estimated to be responsible for 89%-96% and 78%-82% of total infectiousness among children and adults, respectively. Further investigation is required to correlate the substantial variations in viral shedding with heterogeneity in actual transmissibility