State officials suspect the avian flu has killed dozens of wild birds in Boston, Amherst, and Plymouth, and farmers are stepping up precautions.

H5N1 avian influenza continues to spread amid commercial and backyard poultry, and additional cases have been reported in domestic cats

Georgia, the top US chicken-producing state, confirmed its second recent case of bird flu in a commercial poultry flock, putting exports of the meat at risk.

"The Chinese poultry lineage may have experienced more vaccine-driven selection compared to other lineages," the researchers wrote.

H5N1 vaccines have been previously licensed, and millions are in the national stockpile. But even with the news of the country’s first human death due to bird flu, vaccination isn’t yet recommended.