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 cause
huge economic losses as well as sporadic human morbidity and mortality.
Vaccination in poultry has often been reported as being ineffective in preventing
transmission and as a potential driving force in the selection of immune
escape mutants.We conducted transmission experiments to evaluate the transmission
dynamics of HPAI H5N1 strains in chickens vaccinated with high and
low doses of immune escape mutants we have previously selected, and analysed
the data using mathematical models. Remarkably, we demonstrate
that the effect of antigenic distances between the vaccine and challenge strains
used in this study is too small to influence the transmission dynamics of the
strains used. This is because the effect of a sufficient vaccine dose on antibody
levels against the challenge viruses is large enough to compensate for any
decrease in antibody titres due to antigenic differences between vaccine and
challenge strains. Our results showthat at least under experimental conditions,
vaccination will remain effective even after antigenic changes as may be
caused by the initial selection in vaccinated birds.