In subjects who received the 7.5 μ g non-adjuvanted formulation for primary vaccination (N = 12) seroprotection rates as measured by SRH 21 days after the booster vaccination were 66.7% and 83.3%, and 100% and 91.7% of subjects achieved neutralising antibody titres > 20 when tested against the homologous A/ Indonesia and the heterologous A/ Vietnam strain, respectively.