What makes the B3.13 H5N1 genotype more dangerous than earlier strains?: A May 2026 NIH/NIAID study found B3.13 replicates more efficiently in human nasal tissue and suppresses interferon-stimulated gene responses, giving it partial innate immune evasion not seen in historical H5N1 strains.Source: CDC Emerging Infectious Diseases
How many US dairy herds have been infected with B3.13 H5N1?: More than 1,047 confirmed dairy herds across 17 states since March 2024. Idaho alone surged from 1 to 59 quarantined herds in twelve days in May 2026.Source: USDA APHIS
Can B3.13 H5N1 spread through the air in dairy barns?: Yes. An Emory University PLOS Biology study detected B3.13 in submicron aerosol particles in milking-parlour air at 14 California farms, and in exhaled breath from cows before clinical signs appeared.Source: PLOS Biology / Emory University
Why have so few dairy workers caught H5N1 despite widespread cattle infection?: Researchers suggest that 66% of US dairy workers carry pre-existing neutralising antibodies against pdm09 H1N1 that cross-react with B3.13, limiting human cases even as the virus improves its nasal fitness.Source: CDC Emerging Infectious Diseases
What animals besides dairy cattle have tested positive for B3.13?: Domestic cats in Washington and Oregon, alpacas in Idaho, and wild birds across multiple states. The expanding mammalian host range is tracked by USDA APHIS.Source: USDA APHIS

Background

B3.13 is the clade 2.3.4.4b genotype of H5N1 highly pathogenic avian influenza (HPAI) that has dominated US dairy cattle herds since the outbreak began in March 2024. It is distinct from earlier poultry-origin H5N1 strains in its mammalian adaptation profile. A May 2026 study in CDC Emerging Infectious Diseases (DOI: 10.3201/eid3205.260053), authored by NIH/NIAID researchers, found that B3.13 replicates more efficiently in human nasal epithelium cultures than historical H5N1 strains and suppresses interferon-stimulated gene (ISG) responses, enabling partial evasion of the innate immune alarm.

B3.13 belongs to the Eurasian lineage of H5N1 and has been detected across 17 US states and more than 1,047 confirmed dairy herds since March 2024. It was identified in submicron aerosol particles in milking-parlour air at 14 California dairy farms, and in pre-symptomatic milk tank samples, by an Emory University PLOS Biology study published in May 2026.

The ISG suppression finding shifts the pandemic risk posture for B3.13 upward. The two-step mechanistic picture now established — aerosol transmission documented, nasal replication and immune evasion confirmed — means the virus can reach human airways and buy time against the innate immune response before adaptive immunity engages. The principal brake on sustained human spread is thought to be the 66% pre-existing pdm09 H1N1 cross-reactive antibody prevalence among US dairy workers, which may be limiting case counts even as viral fitness improves.

B3.13 is the core H5N1 risk signal tracked on this topic. Idaho's B3.13 outbreak surged from 1 to 59 quarantined herds across four counties in twelve days in May 2026, making Idaho the second-highest state in the national dairy total. The same genotype has been confirmed in domestic cats (Washington, Oregon) and alpacas (Idaho), widening the mammalian host range. The CEPI/Moderna Phase 3 mRNA H5N1 vaccine trial begun in April 2026 targets B3.13 clade strains as the primary vaccine antigen source.

Source Material

USDA APHIS HPAI Dashboard Pathogenesis of HPAI H5N1 B3.13 in Human Airway Epithelium

How the World Sees Them

World Health Organization

The outbreak is serious but is not the next COVID.