Influenza vaccination is the most effective method of preventing influenza and its complications. In the 2009 influenza A (H1N1) pandemic, monovalent strain-specific pandemic vaccines were developed rapidly. However, they were only available in limited supply at the initial stage of the vaccination campaign. Thus, tiered use of vaccines, after careful prioritization and determination of dose per individual, was important to maximize the benefit of the available doses. In this study, the principles and methods of epidemiological evaluation of influenza vaccines were investigated, focusing on the immunogenicity and effectiveness. The results of the study of the 2009/H1N1 pandemic will then be detailed.