In the spring of 2009, a recently emerged novel influenza A (H1N1) virus was first identified in Mexico and USA and it has continued to spread globally. The rapid global spread of a novel influenza A (H1N1) 2009 virus prompted the World Health Organization (WHO), on 11 June 2009, to declare the first influenza pandemic in 41 years. In Taiwan, a clinical study to assess the immunogenicity and safety of Influenza Virus Vaccine, AdimFlu-S (A/H1N1), in healthy volunteers has already been completed. In the previous study, we found that a single 15 mcg HA dose of the AdimFlu-S (A/H1N1) vaccine induces a protective immune response in most adults, including those > 60 years of age (>70%). Our current study aims to follow-up subjects who received Influenza Virus Vaccine, AdimFlu-S (A/H1N1), six months ago. These subjects' serum samples were tested for anti-hemagglutinin (HA) antibodies by hemagglutination inhibition (HAI). The seroconversion is defined as the post-vaccination serum HAI titer had at least 1:40 for subjects who had seronegative pre-vaccination or a four-fold or greater increase in HAI titers for subjects who had seropositive pre-vaccination serum. The immunogenicity of Influenza Virus Vaccine, AdimFlu-S (A/H1N1), in adults after half a year will be analyzed and discussed among the subjects with serum HAI titer had at least 1:40 at least 3 weeks after AdimFlu-S (A/H1N1) injection.