The Anti-Influenza Hyperimmune Intravenous Immunoglobulin Clinical Outcome (FLU-IVIG) study is an international randomized double-blind placebo-controlled trial of anti-influenza hyperimmune intravenous immunoglobulin (IVIG) in individuals hospitalized with influenza (flu) A or B, to determine whether, when added to standard of care treatment, IVIG helps reduce the severity and duration of flu symptoms.