Cases were stratifi ed by presence or absence of any chronic medical conditions in the patients known to increase the risk for infl uenza-associated complications (including moderate to severe developmental delay; hemoglobinopathy, immunosuppressive disorders, asthma or reactive airway disease, diabetes mellitus, history of febrile seizures, seizure disorder, cystic fi brosis, or cardiac, renal, chronic pulmonary, metabolic, or neuromuscular disorders) (11) . Case-patients without chronic medical conditions were classifi ed as "previously healthy." Casepatients who were admitted to an inpatient ward or intensive care unit were classifi ed as "hospitalized." Length of illness was defi ned as the duration of time between the reported date of illness onset and death. Case-patients with length of illness <3 days were classifi ed as having "sudden death." Bacterial co-infection was defi ned as at least 1 positive culture for a bacterial pathogen from a normally sterile site (e.g., blood, cerebrospinal fl uid).