The purpose of this research study is to study the safety and effectiveness of vaccinating individuals who have previously received an avian influenza vaccine derived from one type of H5N1 virus with a vaccine derived from a different type of avian influenza virus. A second reason for this study is to compare responses in people who have received two different but similar types of H5N1 vaccine to the responses in subjects who receive 2 doses of only the H5N1 vaccine used in this study. The information obtained may provide important information into the usefulness of a pre-pandemic vaccination. Participants will include 600 healthy adult volunteers, ages 19 and older, in the United States. Study procedures include: physical exams, vaccination with either a low dose (15 micrograms) or high dose (90 micrograms) of vaccine, blood samples, and maintaining a memory aid to record oral temperatures and side effects. Study participation will be approximately 7 months.