Immunogenicity of a Monovalent Pandemic Influenza A H1N1 Virus Vaccine with or without Prior Seasonal Influenza Vaccine Administration

The immunogenicity of pandemic influenza A H1N1 virus (A/H1pdm) vaccine might be modified by prior seasonal trivalent in-fluenza vaccine (sTIV) administration. We conducted a retrospective analysis of immunogenicity of 243 health care workers (number of sTIV-positive [sTIV] subjects, 216; number of sTIV subjects, 27) by hemagglutination inhibition. There was no significant difference in the ratios of antibody titers of>40 (41.2 % versus 48.1%; P 0.49) and fold increases in geometric mean titer (3.8 versus 4.5; P 0.37). sTIV injected 7 to 10 days prior to A/H1pdm vaccine administration did not interfere with the immunogenicity of the latter. Human infections with pandemic influenza AH1N1 virus (A/H1pdm)were identified in April 2009. The availability of safe and effective vaccines was a critical component of efforts to pre-vent A/H1pdm infection and expansion of the pandemic (1). In Chiba University Hospital (CUH), seasonal trivalent influenza vaccine (sTIV) administrationwas conducted 7 to 10 days prior to A/H1pdm vaccination because the precise date of the initial A/H1pdm vaccine supply was unclear owing to limitations in the manufacturer’s production capacity. There was uncertainty as to whether prior sTIV administra