The purpose of this study is to evaluate the antibody response to each of the three influenza vaccine strains included in the licensed seasonal flu vaccine, as measured by the method of hemagglutination inhibition (HI) and ELISA-based microneutralization (microNT-ELISA) assays. All participants will be divided into 3 groups: participants refused to receive vaccination, those receive either one (week 0) or one more booster vaccination (week 0 and week 4). The investigators will collect serum of participants at the 5th weeks, 9th weeks, and 21th week post vaccination and evaluate the difference of immune response in these 3 groups.
