Neutralizing antibody but not hemagglutination antibody provides accurate evaluation for protective immune response to H5N1 avian influenza virus in vaccinated rabbits.

In order to develop an animal model and an assay method to evaluate protective immune response to H5N1 avian influenza vaccination, H5N1 avian influenza vaccine was prepared. New Zealand rabbits were assigned to receive two doses of vaccine with different hemagglutinin (HA) dosage. The sera from vaccinated rabbits was evaluated to determine antibody titer and specificity using different tested methods including hemagglutination inhibition assay (HI), neutralizing assay (NT), cross-HI assay, cross-single immunodiffusion assay and cross-neutralization assay. The titer of HI antibody from rabbits immunized with different doses of HA were no less than 1:40 among groups 14 days after the first immunization. Whereas the NT antibody titer was less than 1:10 among groups 14 days after the first immunization. NT antibodies can be detected 14 days after the second immunization in rabbits immunized at HA doses higher than 6 μg, and the NT antibody titers were equal to or higher than 1:40. A good concentration-dependent NT antibody response can be detected in the vaccinated rabbits 14 days after the second immunization, and in contrast, no concentration-dependent relationship can be seen for HA antibody. The cross-HI test showed sera from vaccinated rabbits could cross react with influenza A H5N1 virus with the titers higher than 1:40. No cross reaction among different types (influenza A/H1N1 virus, influenza A/H3N2 virus, influenza B virus and influenza A/H5N1 virus) can be detected in the sera using the single immunodiffusion assay and using NT antibody test. This showed NT antibody test was demonstrated as a more accurate assay method for evaluating vaccination and quality of the vaccine than HI antibody test.