Protective efficacy in mice of monovalent and trivalent live attenuated influenza vaccines in the background of cold-adapted A/X-31 and B/Lee/40 donor strains.

Influenza virus continues to take a heavy toll on human health and vaccination remains the mainstay of efforts to reduce the clinical impact imposed by viral infections. Proven successful for establishing live attenuated vaccine donor strains, cold-adapted live attenuated influenza vaccines (CAIVs) have become an attractive modality for controlling the virus infection. Previously, we developed the cold-adapted strains A/X-31 and B/Lee/40 as novel donor strains of CAIVs against influenza A and B viruses. In this study, we investigated the protective immune responses of both mono- and trivalent vaccine formulations in the mouse model. Two type A vaccines and one type B vaccine against A/New Caledonia/20/99 (H1N1), A/Panama/2007/99 (H3N2), and B/Shangdong/7/97 in the background of the A/X-31 ca or B/Lee/40 ca were generated by a reassortment procedure and evaluated for their immunogenicity and protective efficacy. Each monovalent vaccine elicited high levels of serum antibodies and conferred complete protection against homologous wild type virus infection. As compared to the monovalent vaccines, trivalent formulation induced higher levels of type A-specific serum antibodies and slightly lower levels of type B-specific antibodies, suggesting an immunological synergism within type A viruses and an interference in the replication of type B virus. Relatively lower type B-specific immunogenicity in trivalent vaccine formulation could be effectively implemented by increasing the vaccine dose of influenza B virus. These results of immunogenicity, protection efficacy, and immunological synergism between type A vaccines provide an experimental basis for optimal composition of trivalent vaccines for subsequent developments of multivalent CAIVs against seasonal and pandemic influenza viruses.