Rapid Spread of H5N1 D1.1 Avian Influenza in Wild Birds and Humans

Researchers have documented the rapid expansion of a new strain of highly pathogenic avian influenza A(H5N1), identified as genotype D1.1, across North American wild bird populations during the 2024 migratory season. The findings, published in Nature Medicine on April 15, 2026, were based on a comprehensive map of the virus’s spread created through active and passive genomic surveillance.

The study was led by an international collaboration including scientists from St. Jude Children’s Research Hospital, with corresponding author Richard Webby, PhD, from the St. Jude Department of Host-Microbe Interactions. The researchers collected and sequenced samples from birds across the continent to track the geographic penetration of the virus.

Geographic Spread and Emergence

The H5N1 clade 2.3.4.4b viruses first entered North America in 2021, where they began reassorting with local avian influenza viruses. In September 2024, the new reassortant later classified as genotype D1.1 was detected.

According to the researchers, the D1.1 strain first appeared in British Columbia and Alaska. From these locations, the virus moved south and east, following known migratory pathways used by wild birds.

Phylodynamic analysis revealed that the D1.1 viruses formed a monophyletic group. This new genotype spread rapidly across several migratory flyways, eventually displacing earlier A(H5) genotypes in those areas.

The surveillance effort focused heavily on waterfowl, such as geese and ducks, which serve as the natural hosts for influenza.

Human Infections and Risk Assessment

The expansion of the D1.1 genotype in wild birds coincided with detections of the virus in other hosts, including humans. Researchers identified 17 human cases linked to the virus, four of which were classified as severe or fatal.

A critical component of the study involved comparing the viruses found in wild birds with those causing human infections. The researchers searched for mammalian-adaptive markers, which are genetic indicators that a virus may be better suited to infect and spread among mammals.

The study found that the mammalian-adaptive markers detected in the human cases were not present in the viruses isolated from wild birds. Based on this evaluation, the investigators classified the current form of the virus as low risk for human-to-human spread.

Vaccine Efficacy and Public Health

The research team also evaluated whether existing medical countermeasures would remain effective against the evolving strain. They tested candidate vaccine viruses and found that they retained antigenic cross-reactivity with the D1.1 strains.

The scientists concluded that current human vaccine stockpiles will likely remain effective against both the versions of the virus found in wild birds and those causing human infections.

The study highlights the importance of coordinated wildlife surveillance across the United States and Canada to monitor the evolution of highly pathogenic avian influenza. By mapping the continental spread of the D1.1 strain, public health officials can better understand the movement of the virus and its potential impact on both the poultry industry and human health.