Immunogenicity was measured as the percentage of participants who achieved seroconversion or significant increase in single radial hemolysis (SRH) area, against each of the three vaccine strains, three weeks after vaccination (day 22), evaluated using SRH assay.
Seroconversion: proportion of participants with negative pre-vaccination serum and a post-vaccination serum area ≥ 25 mm2. Significant increase: proportion of participants with at least a 50% increase in area from positive pre-vaccination serum. Seroconversion or significant increase: proportion of participants with either seroconversion or significant increase.
The European (Committee for Medicinal Products for Human Use [CHMP]) criterion is met, if percentage of participants achieving seroconversion or significant increase in SRH area is 30% (≥65 years).