This study is being conducted in healthy male and female volunteers. 8-17 year-old identical twins will be randomly assigned to receive a single administration of the 2009-2010 formulation of either seasonal trivalent inactivated influenza vaccine (TIV) or live, attenuated influenza vaccine (LAIV). Twins within a pair will receive different vaccines. 18-30 year-old and 70-100 year-old subjects will receive a single administration of the 2009-2010 formulation of TIV. Blood samples to conduct the assays will be taken at pre-immunization, Day 7-8 and Day 28 post immunization.