- Persons must be willing to provide multiple blood specimens collected by venipuncture each year they are in the study. During each of the first two years of study, blood specimens will be collected three times - at the enrollment visit (immediately prior to administration of vaccine or placebo), at the first follow-up visit 3-5 weeks later, and at the end of the influenza season visit (approximately 4-6 months later); during the third year of the study, blood specimens will be collected twice only, at visits before and after the influenza season.