Because infl uenza-associated deaths in children, but not adults, are nationally reportable in the United States, most cases in this study were pediatric cases reported to CDC through the Infl uenza-associated Pediatric Mortality Surveillance system. This system requires state public health authorities to report to CDC any infl uenza-associated death among persons <18 years old that occurred within their jurisdiction. Information collected by this surveillance system constitutes the primary phenotypic information used in this study and includes underlying health status and chronic medical conditions, infl uenza vaccination status, clinical course and features, and results of microbiologic and virologic testing. Reporting to this surveillance system does not require submission of tissue samples; however, CDC routinely receives tissue samples for a subset of fatal pediatric infl uenza cases for diagnostic confi rmation. For some cases, medical records and autopsy reports provided additional information.