Cross-Reactivity to Highly Pathogenic Avian Influenza H5N1 Viruses after Vaccination with Nonadjuvanted and MF59-Adjuvanted Influenza A/Duck/Singapore/97 (H5N3) Vaccine: A Potential Priming Strategy

Antigenically well-matched vaccines against highly pathogenic avian influenza H5N1 viruses are urgently required. Human serum samples after immunization with MF59 or nonadjuvanted A/duck/Singapore/97 (H5N3) vaccine were tested for antibody to 1997-2004 human H5N1 viruses. Antibody responses to 3 doses of nonadjuvanted vaccine were poor and were higher after MF59-adjuvanted vaccine, with seroconversion rates to A/HongKong/156/97, A/HongKong/213/03, A/Thailand/16/04, and A/Vietnam/1203/04 of 100% (P<.0001), 100% (P<.0001), 71% (P =.0004), and 43% (P =.0128) in 14 subjects, respectively, compared with 27%, 27%, 0%, and 0% in 11 who received nonadjuvanted vaccine. These findings have implications for the rational design of pandemic vaccines against influenza H5.