Key Message
A longitudinal un-controlled study followed participants enrolled in the phase II IVACFLU-A/ H5N1 vaccine trial in Ninh Hoa, Vietnam. The participants tested for anti-A/H5N1 IgG at baseline, 30 months, and 36 months after the complete vaccination to assess antibody status. 84.9% and 52.3% of the population have persistence of at least 1/10 of the A/H5N1 antibodies at 30 months and 36 months, respectively. Antibody persistence was higher in older people but lower in people who had experienced flu-like symptoms in the past or between 2 visits. The result can interpret that remarkable antibody persistence long after vaccination.