Study of the Protective Effect of an Injectable Influenza Vaccine Against Influenza Illness in Adults Under 50 y.o.

Inclusion Criteria: * Good health by history and physical exam * Reliable access to a telephone * Study comprehension and informed consent Exclusion Criteria: * Systolic BP \>/= 140; diastolic BP \>/= 90 * Uncontrolled medical or psychiatric illness (change in last 3 months) * Cancer, or treatment for cancer within 3 years * Cardio-pulmonary disease requiring chronic treatment * Insulin dependent diabetes mellitus * Renal dysfunction (creatinine \>/= 1.7 mg/dL) * Hemoglobinopathies * Clotting disorders that increase the risk of IM injections * Immunosuppressive illnesses or drugs * History of demyelinating disease (esp. Guillian-Barre syndrome) * Employment in professions at high risk for influenza transmission * Household contact with high-risk individuals * Receipt of: a) Immunosuppressive drugs i) systemic glucocorticoids \>/= 10 mg prednisone per day ii) cytotoxic drugs b) Investigational drugs within 30 days before, or planned during, the study c) Blood products within 3 months before, or planned during, the study d) Influenza vaccine within 9 months before the study e) Other vaccines within 30 days before, or planned during, the study * History of: 1. Severe reactions to other influenza vaccines 2. Allergy to egg proteins 3. Sensitivity to mercurials * Pregnancy/ high risk of pregnancy 1. Positive urine pregnancy test before treatment 2. Women of child-bearing potential without credible contraceptive plan