Concerns about bird flu are growing among health officials weeks after the first person in the U.S. died from the virus, and as cases of bird flu grow among animals and humans. Minnesota has no cases of bird flu infection in humans,

"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.