Further evidence of antigenic drift and protective efficacy afforded by a recombinant HVT-H5 vaccine against challenge with two antigenically divergent Egyptian clade 2.2.1 HPAI H5N1 strains.

In this study, we have compared the protection afforded by a recombinant turkey herpesvirus vaccine expressing the H5 gene from a clade 2.2 H5N1 strain (rHVT-H5) and a Mexican-origin H5N2 inactivated vaccine, alone or in combination, against two antigenically divergent H5N1 Egyptian strains isolated in 2007 and 2008. Our results confirm the existence of a major antigenic drift among the Egyptian H5N1 strains such that, although protection against the "classical" 2007 HPAI H5N1 Egyptian strain could be obtained with both types of vaccines, only vaccination with the rHVT-H5 vaccine protected against challenge with the "variant" 2008 HPAI H5N1 Egyptian strain.