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

California has also reported bird flu infections in cats fed raw milk or pet food. In one Los Angeles household, five cats got sick — and two died — after eating two kinds of raw pet food. Samples from one of the two brands, Monarch Raw Pet Food, tested positive for the virus, officials said.

"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.