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The emergence of highly pathogenic avian influenza (HPAI) H5N1 viruses and the risk of a human pandemic have highlighted the need for advance stockpiling of vaccine. Current vaccines may be sub-optimally matched to the actual pandemic virus due to the rapid dissemination and ongoing evolution of avian H5N1 viruses. We report here the evaluation of efficacy of NIBRG-14 vaccine (clade 1 A/Vietnam/1194/2004) against the H5N1 HPAI virus strains circulating in Vietnam. The birds were either vaccinated with a single or booster dose of vaccine by subcutaneous injection; then challenged with three H5N1 HPAI viruses (clade 1, clade 2.3.2.1a and clade 2.3.2.1b) at day 21 post-vaccination (p. v.). The results showed that NIBRG-14 vaccine protected birds from clade 1 and clade 2.3.2.1a infections. Notably, we observed that NIGRG-14 vaccine did not confer protection against clade 2.3.2.1b challenge virus. To get new insights of how H5N1 clade 2.3.2.1b (A/Duck/QuangNgai/1037/11) virus can escape from the host immune response induced by the vaccine, we further analyzed the HA gene - a key virulence determinant of the virus. Amino acid sequence analysis indicated that this virus contained the sequence SPQRERRRK-R/G at the cleavage site in the HA molecule, indicating its high virulence. Additionally, we identified numerous mutations with amino acid substitutions in the hemagglutinin: M226I, I239S located at N-link glycosylation site and 2H, 45N, 53K 120D, 133A and 14N mutations at antigenic site, which can affect receptor specificity as well as viral pathogenicity. Notably, I239S and S133A mutations are unique to A/Duck/QuangNgai/1037/2011, suggesting that it may involve in the virus' ability to evade the host immune system. Taken together, phylogenetic analysis showed that continual mutations in the HA gene may have generated a novel antigenic strain and that probably changed the virulence of the virus and made the H5N1 clade 2.3.2.1b resistant to NIBRG-14 vaccine.