A randomized controlled study to evaluate the immunogenicity of a trivalent inactivated seasonal influenza vaccine at two dosages in children 6 to 35 months of age

The trivalent inactivated influenza vaccine Fluarix™ is licensed in the US for adults and children from 3 years old. This randomized observer-blind study (NCT00764790) evaluated Fluarix™ at two doses; 0.25 ml (Flu-25) and 0.5 ml (Flu-50) in children aged 6–35 months. The primary objective was to demonstrate immunogenic non-inferiority vs. a control vaccine (Fluzone®; 0.25 ml). Children received Flu-25 (n = 1107), Flu-50 (n = 1106) or control vaccine (n = 1104) at Day 0 and for un-primed children, also on Day 28. Serum hemagglutination-inhibition titers were determined pre-vaccination and at Day 28 (primed) or Day 56 (un-primed). Non-inferiority was assessed by post-vaccination geometric mean titer (GMT) ratio, (upper 95% confidence interval [CI] ≤ 1.5) and difference in seroconversion rate (upper 95% CI ≤ 10%). Reactogenicity/safety was monitored. The immune response to Flu-50 met all regulatory criteria. Indicated by adjusted GMT ratios [with 95% CI], the criteria for non-inferiority of Flu-50 vs. control vaccine were reached for the B/Florida strain (1.13 [1.01–1.25]) but not for the A/Brisbane/H1N1 (1.74 [1.54–1.98]) or A/Uruguay/H3N2 (1.72 [1.57–1.89]) strains. In children aged 18–35 months similar immune responses were observed for Flu-50 and the control vaccine. Flu-50 induced a higher response than Flu-25 for all strains. Temperature (≥ 37.5°C) was reported in 6.2%, 6.4%, and 6.6% of the Flu-25, Flu-50, and control group, respectively. Reactogenicity/safety endpoints were within the same range for all vaccines. In children aged 6–35 months, immune responses with Flu-50 fulfilled regulatory criteria but did not meet the pre-defined criteria for non-inferiority vs. control. This appeared to be due to differences in immunogenicity in children aged < 18 months.