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H5N1
Concept

H5N1

Highly pathogenic avian influenza subtype capable of infecting birds, mammals and occasionally humans.

Cambodia supplied four of just seven new confirmed human H5N1 cases worldwide between 16 February and 17 July 2026, Europe's disease agency recorded, while the separate US dairy-cattle pathway held its human count flat at 71.

Last refreshed: 31 July 2026 · Appears in 1 active topic

Key Question

If H5N1 hides in cow udders rather than lungs, is all existing PPE guidance wrong?

Timeline for H5N1

#11 17 Jul

Cambodia, not US herds, makes cases

Pandemics and Biosecurity
#12 16 Jul

Mentioned in: H9N2 case leaves a Chinese man critical

Pandemics and Biosecurity
#8 20 Jun

Why H5N1 hid in cow udders

Pandemics and Biosecurity
#5 29 May

CDC cuts bird-flu reports to monthly

Pandemics and Biosecurity
#4 21 May

Served as the target pathogen for Moderna's mRNA-1018 Phase 3 trial now funded by CEPI

Pandemics and Biosecurity: CEPI rescues Moderna H5N1 vaccine trial
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Background

H5N1 is a highly pathogenic avian influenza subtype that has circulated in US dairy cattle since March 2024 under clade 2.3.4.4b B3.13, alongside a much older, structurally separate poultry-contact pathway that has run in South and Southeast Asia for more than a decade.

A June 2026 Science Advances study led by Suresh Kuchipudi at the University of Pittsburgh explained why the dairy outbreak went undetected in an estimated 1,053 herds across 17 states for roughly two years: cow udder tissue is rich in the N-linked sialic acid receptors the virus targets, while the respiratory tract carries very few, which is why infected cattle present with mastitis rather than flu-like symptoms. Separately, an Emory University study detected the virus in submicron aerosol particles in milking-parlour air across 14 California farms, ahead of any clinical signs in the animals.

Pasteurisation reliably kills the virus, so commercial dairy products carry no risk; raw milk does. PAHO has logged 75 cumulative human cases across the Americas since 2022 with no confirmed community spread, and the virus has been confirmed across dozens of mammalian species and nearly 100 bird species, the breadth that keeps it a persistent pandemic-risk signal rather than a bounded agricultural problem.

Key Issues
Human case count

Cambodia now drives its global case count

Between 16 February and 17 July 2026, global confirmed human H5N1 cases rose from 994 to 1,001, ECDC's week 29 threats report found, a rise of only seven. Cambodia accounted for four of them, its most recent an infant case Cambodian authorities confirmed in early July.

That poultry-contact pathway, running through Cambodia and Bangladesh, is structurally distinct from the US dairy-cattle route that has dominated H5N1 coverage since 2024: it descends from clades that diverged years before the dairy outbreak began, and it is now outpacing the dairy pathway in new confirmed cases for the first time this year.

Vaccine pipeline

Its first Phase 3 vaccine trial began

CEPI-funded work produced the furthest-advanced H5N1 vaccine candidate to date: an mRNA platform that began the first Phase 3 trial for this subtype on 22 April 2026, with first participants dosed a day earlier.

The trial nearly stalled a month later when federal US health authorities withdrew their backing; CEPI stepped in with $54.3 million to keep it running, making the Coalition the sole funder of record for the most advanced mRNA defence against this subtype currently active.

Common Questions

Reference

Why does H5N1 spread so easily in dairy cows but not obviously make them sick?
A June 2026 Science Advances study (University of Pittsburgh, led by Suresh Kuchipudi) found that H5N1 binds N-linked sialic acid receptors abundant in cow udder tissue but near-absent in the respiratory tract. The virus presents as mastitis rather than influenza, bypassing surveillance designed to detect respiratory illness — which is why it went undetected in about 1,053 US dairy herds across 17 states for roughly two years.Source: Science Advances / University of Pittsburgh
Why did H5N1 hide in US dairy cattle for two years?
It binds N-linked sialic acid receptors abundant in cow udder tissue but rare in the respiratory tract, so it caused mastitis-like illness rather than flu, evading surveillance built for respiratory disease.Source: Science Advances
What is the global H5N1 human case count in 2026?
1,001 cumulative cases since 2003 as of 17 July 2026, up from 994 in mid-February, a rise of just seven cases in five months.Source: ECDC
How many H5N1 cases has Cambodia had in 2026?
Cambodia contributed four of the seven new global human H5N1 cases confirmed between 16 February and 17 July 2026, most recently a nine-month-old girl in Phnom Penh.Source: ECDC
How many US dairy farms have H5N1 in 2026?
Approximately 1,053 US dairy herds across 17 states have been infected with H5N1 clade 2.3.4.4b B3.13 as of June 2026. The Idaho detections in May 2026 broke a five-month lull since Wisconsin in December 2025.Source: USDA APHIS
Is there an H5N1 vaccine being developed in 2026?
Yes. Moderna began the first Phase 3 mRNA H5N1 vaccine trial in April 2026, backed by CEPI. Earlier pre-pandemic H5N1 vaccines exist but were based on older strains; this trial targets the clade driving US dairy infections.Source: CEPI / Moderna
Can dairy workers catch H5N1 from breathing air on dairy farms?
Yes. An Emory University PLOS Biology study found H5N1 in submicron aerosol particles in milking-parlour air across 14 California farms, and in the exhaled breath of cows before clinical signs appeared. Standard agricultural PPE was not designed for submicron aerosol exposure from udder rather than respiratory sources.Source: Emory University / PLOS Biology
What happened in Bangladesh and Cambodia with H5N1 in 2026?
Cambodia accounted for four of the seven new human H5N1 cases confirmed globally between 16 February and 17 July 2026 (ECDC's cumulative count rose from 994 to 1,001 in that window), most recently a nine-month-old girl in Phnom Penh, reported 11 July and lab-confirmed 9 July by the National Institute of Public Health and the Institut Pasteur du Cambodge. Cambodia's first 2026 case, confirmed 14 February, falls just outside that window and is not among the seven. The remaining three cases in the window are reported to be split between Bangladesh and India, including a Bangladeshi child who died after household poultry contact, though the exact Bangladesh-India breakdown has not been verified against a primary source. Both countries' cases trace to traditional poultry exposure, not the US dairy-cattle pathway.Source: CIDRAP
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 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
Is H5N1 spreading between people?
No confirmed human-to-human transmission has been recorded; all 2026 cases trace to direct animal contact, either dairy cattle in the US or backyard poultry in Cambodia and Bangladesh.Source: ECDC
Is H5N1 spreading from person to person in the US?
No confirmed human-to-human transmission of H5N1 has been documented in the US. All 71 known US human H5N1 cases are in dairy or poultry workers. PAHO logged 75 cumulative human cases in the Americas since 2022 with no evidence of community spread.Source: CDC / PAHO
Source Material
CDC H5 Bird Flu Situation SummaryInfluenza A virus subtype H5N1