Background
Understanding factors affecting the infectiousness of influenza cases is crucial for disease prevention and control. Viral shedding is expected to correlate with infectiousness of cases, but it is strongly associated with age and the presence of symptoms.

Methods
To elucidate this complex interplay, we analyze with an individual-based household transmission model a detailed household transmission study of influenza with 442 households and 1710 individuals from 2008 to 2017 in Hong Kong, to characterize the household transmission dynamics and identify factors affecting transmissions.

Results
We estimate that age, fever symptoms, and viral load were all associated with higher infectiousness. However, by model comparison, the best model included age and fever as factors affecting individual infectiousness, and estimates that preschool and school-aged children were 317% (95% credible interval [CrI], 103%, 1042%) and 161% (95% CrI, 33%, 601%) more infectious than adults, respectively, and patients having fever had 146% (95% CrI, 37%, 420%) higher infectiousness. Adding heterogeneity on individual infectiousness of cases does not improve the model fit, suggesting these factors could explain the difference in individual infectiousness.

Conclusions
Our study clarifies the contribution of age, symptoms, and viral shedding to individual infectiousness of influenza cases in households.