After initial hesitation about using a wide-scale program of poultry vaccination, some European and Asian countries have begun vaccination. Inactivated vaccines are widely used in poultry but lack of critical potency testing, standardization, and quality control has led to variable and suboptimal immune responses. Moreover, a legitimate concern remains that the fowl vaccinated by attenuated live viruses may survive the disease but still carry the virus; thus, they would continue to spread infl uenza (H5N1) silently at the fl ock level (8) or to humans who come into contact with them. Vaccination that resulted in low levels of seroconversion facilitated the emergence of the Fujian-like sublineage of infl uenza (H5N1) in poultry (9) .