Serum samples from 77 healthy volunteers, matched before and after vaccination (1-4 months), had been collected and stored at a single center, from a dose-escalating clinical trial (Clin-icalTrials.gov identifier: NCT00383071) on an inactivated H5N1 vaccine, conducted at the National Institute of Allergy and Infectious Diseases, National Institutes of Health [8] . The vaccine (manufactured by Sanofi Pasteur) used in the trial was a monovalent, inactivated subvirion H5N1 vaccine (rgA/ Vietnam/1203/04 X A/PR/8/34). The study was conducted in accordance with institutional review board-approved protocol.