The Advisory Committee on Immunization Practices (ACIP) has in recent years recommended that all healthy children aged 6 through 59 months receive influenza vaccine. Because of the growing emphasis on influenza immunization of infants and young children, Center for Biologics Evaluation and Research (CBER) has expressed interest in receiving sera from children who have been vaccinated with the current formulation of Fluzone® vaccine. These sera will be tested to evaluate each individual's immune response to the current formulation of Fluzone® vaccine and will also be used to evaluate circulating influenza strains in order to support formulation recommendations for the subsequent year (particularly for influenza B strains, which predominantly affect the young).