Although Hong Kong has reported H5N1 in wild birds each year, there have been no H5N1 positive poultry reports in the past five years. The latest outbreak is of concern because the chickens are asymptomatic. It is unclear if the H5N1 shedding by asymptomatic chickens is due to partial immunity due to a poor match between the vaccine and H5N1, adaptive changes by the chickens, or changes by the virus.
However, one of the arguments against H5N1 poultry vaccinations is viral shedding by poultry that appear healthy. The silent spread of H5N1 can quickly lead to a much larger genetic reservoir, creating conditions for more rapid evolution. The asymptomatic infections also increase potential for an expanded geographical reach. More
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