1. Supplemental Digital Content 1 Hemagglutination-inhibition (HI) antibodies against A/New Caledonia/20/99 [H1N1], A/Wisconsin/67/2005 [H3N2] and B/Malaysia/2506/2004 for study vaccine (Fluarix®) and control vaccine (Fluzone®) by age group and previous vaccination history (ATP cohort for immunogenicity) PR1= Previously vaccinated children <3 years PR2 = Previously vaccinated children 3-<5 years UN1 = Children not previously vaccinated <3 years UN2 = Children not previously vaccinated 3-<5 years Fluarix = Fluarix ® 0.25 ml for children <3 years and Fluarix ® 0.5 ml for children 3-<5 years Fluzone = Fluzone ® 0.25 ml for children <3 years and Fluzone ® 0.5 ml for children 3-<5 years PRE = Pre-vaccination dose 1 (Day 0) POST = Post-vaccination timepoint (Day 28 for children previously vaccinated or Day 49 for children not previously vaccinated) GMT = Geometric mean titer (i.e. the anti-log of the mean of the log titer transformations) 95% CI = 95% confidence interval ATP: According-to-protocol; HI: Hemagglutination-inhibition

2. PR1= Previously vaccinated children <3 years PR2 = Previously vaccinated children 3-<5 years UN1 = Children not previously vaccinated <3 years UN2 = Children not previously vaccinated 3-<5 years Fluarix = Fluarix ® 0.25 ml for children <3 years and Fluarix ® 0.5 ml for children 3-<5 years Fluzone = Fluzone ® 0.25 ml for children <3 years and Fluzone ® 0.5 ml for children 3-<5 years PRE = Pre-vaccination dose 1 (Day 0) POST = Post-vaccination timepoint (Day 28 for children previously vaccinated or Day 49 for children not previously vaccinated) GMT = Geometric mean titer (i.e. the anti-log of the mean of the log titer transformations) 95% CI = 95% confidence interval ATP: According-to-protocol; HI: Hemagglutination-inhibition

3. PR1= Previously vaccinated children <3 years PR2 = Previously vaccinated children 3-<5 years UN1 = Children not previously vaccinated <3 years UN2 = Children not previously vaccinated 3-<5 years Fluarix = Fluarix ® 0.25 ml for children <3 years and Fluarix ® 0.5 ml for children 3-<5 years Fluzone = Fluzone ® 0.25 ml for children <3 years and Fluzone ® 0.5 ml for children 3-<5 years PRE = Pre-vaccination dose 1 (Day 0) POST = Post-vaccination timepoint (Day 28 for children previously vaccinated or Day 49 for children not previously vaccinated) GMT = Geometric mean titer (i.e. the anti-log of the mean of the log titer transformations) 95% CI = 95% confidence interval ATP: According-to-protocol; HI: Hemagglutination-inhibition