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

Highly pathogenic avian influenza (HPAI) H5N1 epidemics in poultry causehuge economic losses as well as sporadic human morbidity and mortality.Vaccination in poultry has often been reported as being ineffective in preventingtransmission and as a potential driving force in the selection of immuneescape mutants.We conducted transmission experiments to evaluate the transmissiondynamics of HPAI H5N1 strains in chickens vaccinated with high andlow doses of immune escape mutants we have previously selected, and analysedthe data using mathematical models. Remarkably, we demonstratethat the effect of antigenic distances between the vaccine and challenge strainsused in this study is too small to influence the transmission dynamics of thestrains used. This is because the effect of a sufficient vaccine dose on antibodylevels against the challenge viruses is large enough to compensate for anydecrease in antibody titres due to antigenic differences between vaccine andchallenge strains. Our results showthat at least under experimental conditions,vaccination will remain effective even after antigenic changes as may becaused by the initial selection in vaccinated birds