Antigenic and Genetic Diversity of Highly Pathogenic Avian Influenza A (H5N1) Viruses Isolated in Egypt

Abstract

Highly pathogenic avian influenza A virus (H5N1) has diverged antigenically and genetically since its initial detection in Asia in 1997. Viruses belonging to clade 2.2 in particular have been reported in numerous countries with the majority occurring in Egypt. Previous reports identified antigenic similarities between viruses belonging to clade 2.2. However, poultry and human viruses isolated in northern Egypt during 2007 and 2008 were found to be antigenically distinct from other clade 2.2 viruses from this country. Genetic analysis of the hemagglutinin revealed a high degree of nucleotide and amino acid divergence. The antigenic changes in Egyptian viruses isolated during 2007-08 necessitated that two of these strains be considered as potential H5N1 pre-pandemic vaccine candidates.

Role of vaccination-induced immunity and antigenic distance in the transmission dynamics of highly pathogenic avian influenza H5N1

“…Our experimental data show that with a low vaccine dose the large antigenic distances that occur between the most extremely different isolates of the same H5 HA type, i.e. H5N1 R65 compared to H5N2 (80.88 or 2 6.34 HIU as measured in [22]) can play a sufficient role in changing the vaccinated population from not allowing transmission (R , 1) to allowing transmission (R . 1).…”

“…Indeed, many field studies have attributed the failure of vaccination to prevent infection and transmission of HPAI strains to-among other things-antigenic distances between the vaccine and the circulating field strains, and called for constant vaccine updating, the way it is done for human influenza vaccines [6,7,14,20]. These studies show that use of vaccines that are antigenically distant to the circulating strains led to lack of protection due to low HI titres against the challenge strains.…”

Increased substitution rate in H5N1 avian influenza viruses during mass vaccination of poultry

“…Circulation of H5N1 HPAI virus in Egypt was firstly identified in 2005, and was confirmed in each of the subsequent years [18]. However, the impact on disease control of the vaccination in Egypt has been very limited, and a recent assessment study highlighted substantial weaknesses in the vaccination programs in Egypt [19].…”

“…Secondly, vaccination facilitates "silent" spread of the virus, because vaccinated birds usually show no severe clinical signs if they are infected with the virus, and thus farmers may relax their vigilance and relevant biosecurity measures [24]. Furthermore, vaccine-resistant influenza viruses, which reduce the efficacy of mass vaccination programs, have been reported to have emerged in China, Egypt, and Indonesia [6,18,25].…”

“…The HA1 subunit is the major surface antigen, a target of neutralizing antibodies produced during infection or vaccination, and substitutions in the HA1 subunit can result in vaccine-resistant influenza viruses [28,29]. In effect, as far as we know, substitutions in the HA1 subunit play a much more important role than those in the HA2 subunit in vaccine escape [18,25,[30][31][32]. In this sense, the HA2 subunit serves as a control in this study when analyzing the substitution rate of the HA1 subunit.…”