China introduced bivalent H5/H7 vaccination for supply flocks following repeated zoonotic outbreaks of H7N9 avian influenza among consumers purchasing chicken at wet markets. The recombinant vaccines were directed against H5N1 and H7N9.

The vaccination program successfully reduced the prevalence of low pathogenicity H7N9 among flocks supplying wet markets with a corresponding decline in the incidence rate of human infections.

An unfortunate consequence of the program was the emergence of highly pathogenic H7N9 strains that now circulate among flocks in northern China.  This is attributed to mutations induced by vaccination.  It is evident that it is necessary to monitor the emergence of new strains of virus following initiation of any any avian influenza vaccination program and to modify vaccines in response to emergence of variants.