Since 2021, experts have seen bird flu — also called H5 influenza or highly pathogenic avian influenza (HPAI) — spreading "fairly quickly" across the wild and domestic bird populations and, now, among dairy cattle, Martinello tells TODAY.com.

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.

Since the summer, the U.S. has begun preparing about 10 million doses of a vaccine that they think will be effective against H5N1 bird flu. Federal officials have elected not to u

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