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.

Moderna has been testing an mRNA candidate for influenza viruses like the H5 and H7 strains that are seen as pandemic threats. Others, including GSK and Pfizer, are also at work on similar shots.

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