Mapping the antigenic evolution of an H5N1 clade 2.2 A/chicken/ Egypt/1709-1/07 and the drift variant A/chicken/Egypt/1709-6/08 by a sensitive pseudotyped-based assay

The propensity of influenza virus to mutate and evolve into new types with divergent antigenic determinants is an important challenge in the control of influenza infection. In Egypt, an H5N1 avian virus belonging to clade 2.2 was notified in February 2006 and the country has faced a huge number of outbreaks since. Subsequently, to control infection in poultry and prevent the risk of human exposure, an inactivated vaccine containing an H5 virus belonging to a different lineage to the Eurasian H5N1 (H5N2/Mexico) was employed. Despite this, the continuous circulation of the virus and the mass vaccination programme has resulted in progressive genetic evolution and significant antigenic drift with multiple mutations near the major antigenic sites leading to the emergence of a new strain grouped within clade 2.2.1. To establish whether the administration of the H5N2/Mexico vaccine is likely to provide cross-protective antibody responses against different clades, lentiviral pseudotypes derived from H5N1 HPAI viruses (A/Egypt-1709-01/2007) and the antigenic drift variant (A/ Egypt-1709-06/2008) were constructed and used in pseudotype-based neutralisation (pp-NT) assays. Overall results indicated a significantly lower level of antigenic cross-reactivity of post vaccine sera against the pseudotyped-particles bearing the HA of the 1709-06 drift variant