The findings in this report are based, in part, on contributions provided by E Lewis, MPH, SK Greene, PhD, R Yin, MS, E Weintraub, MPH, P Ray, MPH, B Fireman, MS, R Baxter, MD, L Lyon, MS, S Black, MD, J Baggs, PhD, TA Lieu, MD, Vaccine Safety Datalink Rapid Cycle Analysis Team. H Izurieta, MD, J Beeler, MD, R Ball, MD, F Houn, MD, M Braun, MD, Center for Biologics Evaluation and Research, Food and Drug Admin, Rockville, Maryland. S Jacobsen, MD, PhD, Div of Research and Evaluation, Southern California Kaiser Permanente, Los Angeles, California. Merck & Co., Inc., staff members, Whitehouse Station, New Jersey. C Chesley, Div of Viral Diseases, National Center for Immunization and Respiratory Diseases, CDC.