After WHO declared H1N1 pandemic, global vaccination was carried out immediately after much research. However, the data on long-term immunogenicity were lacking. We aimed to investigate the long-term immunogenicity of different H1N1 vaccine dosage groups 24 weeks after vaccination by a randomized clinical trial. A total of 218 participants were stratified into adult (≤60 years old) and elderly (>60 years old) groups. The adults were randomized in a 1:1:1 ratio. The first group received a single dose of vaccine with 15 μg hemagglutination antigen (HA). The other two groups received two doses with 15 μg or 30 μg HA triweekly. The elderly were randomized 1:1 for two doses of 15 or 30 μg HA. We evaluated serologic responses at prevaccination and weeks 3, 6, and 24. We also examined possible associated factors of immunogenicity by multivariate logistic regression analyses. At week 24, seroprotection (anti-HA antibody level ≥ 1:40) remained at 76.8% and 46.2% in the adult and elderly groups, respectively. The adult group had a higher seroprotection rate (odds ratio of 2.98, 95% confidence interval [CI]: 1.21 to 7.36) than the elderly group. There was no statistical difference in seroprotection and seroconversion rates between different adult and elderly dosage groups. Lower immunogenicity in the elderly than in the adults 24 weeks after the vaccination was observed. However, there was no statistically significant difference among different dose groups. Therefore, we suggest only a single vaccination dose of 15 μg HA for adults and two doses of 15 μg HA for the elderly in the future.