This study will evaluate differences in health outcomes and healthcare resource utilization between moderate-to-severe influenza compared to mild influenza as defined by Jain et al 2013 in children aged 6 to 35 months in the US. Secondary objectives: To evaluate differences in health outcomes and healthcare resource utilization between moderate-to-severe influenza compared to mild influenza as defined by Jain et al 2013 in children aged 6 months to 18 years in the US, and to evaluate differences in health outcomes and healthcare resource utilization between moderate-to-severe influenza compared to mild influenza as defined by Jain et al 2013 in children aged 6 to 12 months in the US.