On 12 October 2026, the first case of a human H5N1 infection in 2026 was detected in Washington State. The sequence (EPI_ISL_205571770) clusters with recent samples from Idaho and Utah, carrying the same mutation in the NA segment.

Clade 2.3.4.4b of the highly pathogenic avian influenza (HPAI) virus causing outbreaks in wild and domestic birds around the world continues to spread in dairy cows, poultry and other animals across the United States. Since April 2024, the U.S. CDC confirmed 72 human cases through genome sequence analysis. Data in some of these cases showed the amino acid substitution NA-S247N known to slightly reduce susceptibility to the neuraminidase inhibitor oseltamivir in laboratory tests. In one case, a different change in the polymerase acidic (PA) protein was detected.

In March 2025, the U.S. CDC had released specimen sequence data from the first human case in Ohio, belonging to genotype D1.3. Previously the CDC had released data from the first human fatality involving a patient from Louisiana who had been exposed to non-commercial backyard poultry and wild birds.