Inclusion Criteria - 1. Children ages 2 - 7.99 years with a diagnosis of Hb SS or Hb Sß0 thalassemia, documented by hemoglobin electrophoresis and a complete blood count (CBC). 2. Influenza vaccination during the previous year or intended before the upcoming flu season. 3. Evidence of past infection with, or immunization against, varicella. 4. Negative pregnancy tests in girls of childbearing potential. 5. Informed consent signed by the parent or legal guardian.
Exclusion Criteria - 1. Prior history of overt stroke or cerebral hemorrhage. 2. Known history of allergic reaction to aspirin. 3. History of Reye's syndrome 4. Diagnosis of G-6-PD deficiency or von Willebrand's disease 5. Prolongation of the bleeding time or abnormal closure time, prothrombin time (PT), or partial thromboplastin time (PTT). 6. Active gastrointestinal (GI) bleeding or a history of GI bleeding. 7. Hepatic disease (AST or ALT >2x upper limit of normal, Direct bilirubin > 1.5 mg/dL) or renal disease (creatinine >2x upper limit of normal or 2 mg/dl, whichever is smaller). The exclusion criteria laboratory study ranges have been specified as greater than 2 times the upper limit of normal. 8. Hypertension (BP >95% for age and height). 9. Current treatment with chronic transfusion therapy. 10. Evidence of hemorrhage on MRI. 11. A mean TCD velocity > 200 cm/sec. in the middle cerebral artery (MCA) or internal carotid artery (ICA). 12. Evidence of Moyamoya syndrome on MRA. 13. Evidence of pregnancy. 14. Evidence of an inability to comply with testing procedures. 15. Inability to provide informed consent.