Phase I - Influenza-vaccine naïve children (n = 48) aged >= 6 to < 24 months will be randomized in a 1:1:1 ratio to the following three study arms, each to receive two doses on days 0 and 28 of trivalent inactivated influenza (INF) vaccine (Vaxigrip®, Sanofi Pasteur, Lyon, France) into the left thigh (< 12 months) or left deltoid (≥ 12 months):