Immunogenicity and Reactogenicity of an Inactivated Quadrivalent Influenza Vaccine Administered Intramuscularly to Children 6 to 35Months of Age in 2012–2013: A Randomized, Double-Blind,

Background. Influenza attack rates are high in 6- to 35-month-old children; vaccines containing both lineages of influenza B (Yamagata and Victoria), in addition to the H3N2 and H1N1 antigens, may improve protection rates. Methods. In a randomized double-blind controlled trial, the immunogenicity and reactogenicity of an inactivated quadrivalent influenza vaccine (QIV) and a trivalent control vaccine (TIV) were assessed. Results. Six hundred one children (QIV, n = 299; TIV, n = 302) were enrolled at 8 sites in 3 countries. The primary immunogenicity objective was met: the lower limit (LL) of the 2-sided 95 % confidence interval (CI) for the seroconversion rate in QIV recipients ranged from 66.6 % to 81.3%, which was 40 % against all 4 strains. The immunogenic superiority of the additional B/Victoria strain in the QIV compared to that in the TIV was confirmed: the LL of the 2-sided 95 % CI of the geometric mean titer ratio (QIV/TIV) (6.28 [95%CI, 5.32–7.41]) was greater than 1.5, and the LL of the 2-sided 95 % CI for the difference in the seroconversion rate (QIV – TIV) (64.19 % [95%CI, 57.65%–69.95%]) was greater than 10%. Injection-site pain and irritability/fussiness were th