† seroprotection rate: proportion of subjects with haemagglutination inhibition (HI) titre ≥ 1:40; seroconversion rate: proportion of subjects who were either seronegative at pre-vaccination and have a protective post-vaccination titre of ≥ 1:40, or who were seropositive at pre-vaccination and have a 4- fold increase in titre; seroconversion factor: ratio of the post-vaccination geometric mean titre (GMT) and the pre-vaccination GMT.