Is H5N1 bird flu spreading in US dairy farms in 2026?: Yes. H5N1 was confirmed in Idaho dairy cattle in May 2026, the first US dairy detection since Wisconsin in December 2025. An Emory study found the virus in submicron aerosol particles in milking-parlour air.Source: CIDRAP
Can dairy workers catch H5N1 from breathing air on dairy farms?: An Emory University PLOS Biology study detected H5N1 in submicron aerosols in milking-parlour air on infected farms. Current CDC guidance focuses on contact precautions but does not address aerosol respiratory exposure.Source: PLOS Biology / Emory
How many people have caught H5N1 from cows in the US?: 71 human H5N1 cases in the US have been recorded, all in dairy or poultry workers, with no community spread. PAHO logs 75 cumulative cases in the Americas since 2022 across 5 countries.Source: PAHO / CDC
Is there an H5N1 vaccine being developed in 2026?: Yes. CEPI announced on 22 April 2026 that Moderna had started the first Phase 3 trial of an mRNA H5N1 vaccine, the furthest any H5N1 mRNA candidate has reached in clinical development.Source: CEPI
What happened in Bangladesh and Cambodia with H5N1 in 2026?: A child in Bangladesh died in February 2026 after household poultry contact; Cambodia logged 3 human H5N1 cases in 2026. Both involve traditional poultry exposure, not the dairy-cattle pathway seen in the US.Source: CIDRAP

Background

H5N1 is a subtype of highly pathogenic avian influenza A (HPAI) virus that has circulated in wild birds and poultry since the late 1990s and represents one of the most closely watched pandemic-risk pathogens. The clade 2.3.4.4b has been responsible for a globally unprecedented spread across wild bird and mammalian species since 2021. In humans, H5N1 has historically had a case-fatality rate above 50% in confirmed clinical cases, though outbreak ascertainment bias makes the true rate uncertain. The virus has infected dairy cattle in the US since March 2024, a mammalian transmission pathway not previously seen at scale. WHO monitors H5N1 via the GISRS network; USDA tracks animal-side detections through the APHIS HPAI dashboard.

H5N1 (clade 2.3.4.4b B3.13) has been detected in US dairy cattle in Idaho as of May 2026, the first US dairy detections since Wisconsin in December 2025, breaking a five-month lull. An Emory University PLOS Biology study found the virus in submicron aerosol particles in milking-parlour air across 14 California farms, and in the exhaled breath of cows before clinical signs appeared. The clade in the air samples matches the one behind 71 known US human H5N1 cases. PAHO logged 75 cumulative human cases in the Americas since 2022, with no new human cases since November 2025 as of its March 2026 update. Moderna began the first Phase 3 H5N1 mRNA vaccine trial in April 2026 via CEPI.

Source Material

CDC H5 Bird Flu Situation Summary Influenza A virus subtype H5N1

How the World Sees Them

United States

71 human cases concentrated in dairy and poultry workers; the Idaho detection after a five-month lull breaks the assumption that the US outbreak had peaked, and the Emory aerosol finding puts CDC guidance under pressure.

Americas (via PAHO)

75 cumulative human cases across 5 countries since 2022; PAHO's March 2026 count froze the US figure at 71 but did not capture the April-May 2026 period.

Bangladesh and Cambodia

Human cases in both countries involve traditional backyard poultry contact with a clade circulating locally since around 2011; structurally different from the US dairy-cattle pathway but both count toward the global H5N1 human caseload.

Global health surveillance

H5N1 detected in 37 mammalian species and 94 bird species per PAHO March 2026; the breadth of host range is what makes this a persistent pandemic-risk signal rather than a bounded agricultural problem.