Combined use of live-attenuated and inactivated influenza vaccines to enhance heterosubtypic protection Li-meng Yan, Leo LM Poon Centre of Influenza Research, School of Public Health, The University of Hong Kong, Hong Kong The limited protection of current commerical vaccines necessitates the investigation of novel vaccine strategies for unpredictable outbreaks. To investigate the feasibility of using vaccines derived from Group 1 influenza A virus to induce broadly cross-reactive immune responses against multiple influenza subtypes, we tested a panel of sequential 4-dose immunization regimens in mice. Mice were treated with inactivated (seasonal H1N1/A/Brisbane/59/07, pandemic H1N1/A/California/09/2009, and highly pathogenic H5N1/A/VN/1203/04) and vaccinia virus-based H5N1 live-attenuated (Wyeth/IL-15/5Flu) vaccines in different combinations. Wyeth/IL-15/5Flu is a novel pentavalent vaccine, expressing HA, NA and NP proteins from H5N1/A/Vietnam/1203/2004, M1 and M2 proteins from H5N1/A/CK/Indonesia/PA/2003 virus, and human IL-15 as a molecular adjuvant. Mice were then challenged by a lethal dose of either Group 1 (H1N1/A/Puerto Rico/8/1934) or Group 2 (mouse-adapted H3N2/A/Hong Kong/1/1968, HAPI H7N7/A/Netherlands/219/2003) influenza virus. All studied sequential 4-dose vaccinations could induce some degrees of heterosubtypic protection in mice. Amongst all these regimens, the combined use of inactivated and live-attenuated vaccines could achieve the best heterologous protection. The results of immune profiles from such regimens provide related information about synergistic effect of combining different vaccine platforms for universal vaccinne development and optimization. Further research will focus on developing alternative vaccine strategies, including the use of different kinds of viral immunogens or adjuvants, for universal protection against influenza.