D1.1 H5N1 spread continent-wide via migratory birds
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- St. Jude Children's Research Hospital led an international collaboration that mapped the D1.1 H5N1 strain as it first appeared in Alaska and British Columbia, then spread south and east along known migratory pathways in 2024.
- D1.1 was identified as the cause of every severe H5N1 human infection reported in North America starting in 2024; researchers linked these previously scattered cases to the strain's spread through wild bird populations.
- The study, published in Nature Medicine, classified the D1.1 viruses as low risk for human-to-human transmission in their current form, noting they lacked the major mutations that enable efficient human infection.
- The D1.1 strain carries a mutation conferring resistance to a common antiviral drug, but existing candidate vaccines showed significant cross-reactivity against it, suggesting current stockpiles would likely be effective.
- H5N1 first entered North America in 2021, causing widespread harm to wild birds and the poultry industry; the D1.1 strain emerged as a novel variant detected by the St. Jude group in late 2024.
Why it matters: Researchers now have a working model for tying scattered human H5N1 infections to wild-bird migration patterns, and current vaccine stockpiles are likely to remain effective against the dominant D1.1 strain — but the virus's resistance to a common antiviral limits treatment options for the few severe cases that do occur.
