Seroconversion (serocon.) is defined as negative pre-vaccination serum (titer \<10 for HI \[Haemagglutination Inhibition\], area ≤4 mm\^2 for SRH \[Single Radial Haemolysis\]) / positive post-vaccination titer (titer ≥ 40 for HI, area ≥ 25 mm\^2 for SRH). Significant increase in antibody titer is defined as at least a fourfold increase from non-negative pre-vaccination serum (HI ≥ 10) or at least 50% increase in the SRH area. Seroprotection is defined as a HI titer ≥40 and a SRH area ≥25 mm\^2.

Geometric mean Ratio (GMR) was calculated for the haemagglutination inhibition (HI), microneutralization (MN) and single-radial haemolysis (SRH) result as well as the associated 95% confidence intervals. GMR was calculated as 21 days after second and third vaccinations over day 1.

seroconversion: negative pre-vaccination serum (HI titer \<10, SRH area =\<4 mm\^2)/positive post-vaccination titer (HI titer =\>10) or at least 50% increase in the SRH area. Seroprotection is defined as a HI titer ≥40 and a SRH area ≥25 mm\^2.

seroconversion (serocon.): negative pre-vaccination serum (HI titer \<10, SRH area =\<4 mm\^2)/positive post-vaccination titer (HI titer =\>10) or at least 50% increase in the SRH area. Seroprotection is defined as a HI titer ≥40 and a SRH area ≥25 mm\^2.

seroconversion (serocon.): negative pre-vaccination serum (HI titer \<10, SRH area =\<4 mm\^2)/positive post-vaccination titer (HI titer =\>10) or at least 50% increase in the SRH area. Seroprotection is defined as a HI titer ≥40 and a SRH area ≥25 mm\^2.

For each vaccine group, the least squares GMRs were calculated for the haemagglutination inhibition (HI) results as well as the associated 95% confidence intervals. GMR was calculated over day 1.

For each vaccine group, the least squares GMRs were calculated for the haemagglutination inhibition (HI) results for each time point of the study, as well as the associated 95% confidence intervals. GMR was calculated over day 1.

For each vaccine group, the least squares GMRs were calculated for the haemagglutination inhibition (HI) results for each time point of the study, as well as the associated 95% confidence intervals. GMR was calculated over day 1.