Antibody persistence after vaccination with the 7.5 µg non-adjuvanted formulation of CELVAPAN (strain A/ Vietnam/ 1203/ 2004) has been evaluated in two clinical studies in adults aged 18 – 59 years (N=285) and in one clinical study in elderly subjects aged 60 years and above (N=258) up to 6 months after the start of the primary vaccination series.