Seroprotective titres as determined by SRH assay against the homologous vaccine strain (A/ Vietnam/ 1203/ 2004) were observed in 65.5% of subjects aged 18 – 59 years and in 59.4% of subjects aged 60 years and older at 21 days after a booster vaccination with the 7.5 μ g dose of the A/ Vietnam strain vaccine.