Immune response in domestic ducks following intradermal delivery of inactivated vaccine against H5N1 highly pathogenic avian influenza virus adjuvanted with oligodeoxynucleotides containing CpG motifs

Ducks are a natural reservoir for H5N1 highly pathogenic avian influenza (HPAI) viruses, which produces a range of clinical outcomes from asymptomatic infections to severe disease with mortality. Vaccination against HPAI is one of the few methods available for controlling avian influenza virus (AIV) infection in domestic ducks; therefore, it is necessary to improve vaccine efficacy against HPAI in domestic ducks. However, few studies have focused on enhancing the immune response by testing alternative administration routes and adjuvants. While attempting to maximize the efficacy of a vaccine, it is important to select an appropriate vaccine delivery route and adjuvant to elicit an enhanced immune response. Although several studies have indicated that the vaccination of ducks against HPAI viruses has offered protection against lethal virus challenge, the immunogenicity of the vaccine still requires improvement. In this study, we characterized the immune response following a novel vaccination strategy against H5N1 HPAI virus in domestic ducks. Our novel intradermal delivery system and the application of the cytosine–phosphodiester–guanine (CpG) oligodeoxynucleotide (ODN) adjuvant allowed us to obtain information regarding the sustained vaccine immunity. Compared with the intramuscular route of vaccination, the intradermal route resulted in higher antibody titer as well as lower antibody deviation following secondary vaccination. In addition, the use of a CpG–ODN adjuvant had a dose-sparing effect on antibody titer. Furthermore, when a high dose of antigen was used, the CpG–ODN-adjuvanted vaccine maintained a high mean antibody titer. This data demonstrates that intradermal immunization combined with administration of CpG–ODN as an adjuvant may be a promising strategy for improving vaccine efficacy in domestic ducks.