Fig. 1: Study outline and serological responses.

a Study outline. Ninety children, 15–24 months old, were randomized into three cohorts, A, B, and C, illustrated in blue color. Each infant received two injections of MF59-adjuvanted trivalent inactivated seasonal influenza vaccine, 28 days apart. Blood was sampled from each infant at three time points. Each infant from all three cohorts was sampled at day 56 of the study for the measurement of HAI titers. In addition, children in Cohort A were sampled at the pre-vaccination baseline (day −7 to day 0 of the study) and day 1 post prime vaccination. Children in Cohort B were sampled at day 28 (pre-boost baseline) and day 29 (1 day post boost vaccination). Children in Cohort C were sampled at day 28 and day 31 (pre-boost baseline and day 3 post boost vaccination, respectively). HAI titers were measured in all samples. Immunoprofiling of responding cellular subsets and transcriptional analysis was performed at the pre-vaccination baseline (Cohort A), day 1 post prime (Cohort A), Pre-boost baseline (Cohorts B and C), day 1 post boost (Cohort B) and day 3 post boost (Cohort C). In addition, 30 young (age 25–40 years) healthy adult volunteers received a single dose of the same vaccine (Cohort D, illustrated in orange color). Blood was sampled from all adult volunteers at the pre-vaccination baseline (day 28), day 29, and day 31 of the study (days 1 and 3 post vaccination), and day 56 of the study. HAI titers were measured at all time points, and immunoprofiling of responding cellular subsets and transcriptional analysis was performed at the baseline and days 1 and 3 post vaccination. b Serological responses. The changes in HAI titers against each one of the vaccine strains: H1N1 (A/California), H3N2 (A/Switzerland) and B (B/Phuket) are illustrated in each column. Each row corresponds to one of the four cohorts of the study. Each line represents a subject; bold lines represent mean changes in HAI titers across all subjects in a cohort.