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H5N1
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Concept

H5N1

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

Between 16 February and 17 July, Cambodia accounted for four of seven newly confirmed human H5N1 cases worldwide, while the United States dairy-associated human count remained at 71.

Last refreshed: 22 September 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

#14 27 Sept

Infected 11 people worldwide in 2026

Pandemics and Biosecurity: Cambodia confirms sixth human H5N1 case
#14 18 Sept

Mentioned in: WHO talks on pathogen sharing fail again

Pandemics and Biosecurity
#13 13 Aug

Mentioned in: Australia releases its first H5N1 genome

Pandemics and Biosecurity
#13 6 Aug

Mentioned in: MERS clade B found in Nile Basin camels

Pandemics and Biosecurity
#11 17 Jul

Cambodia, not US herds, makes cases

Pandemics and Biosecurity
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Background

H5N1 is a highly pathogenic avian influenza subtype that infects birds and mammals, with occasional human infection. Its broad animal host range and ability to cross species make it a continuing surveillance concern rather than a disease confined to one farm sector.

The subtype has followed more than one exposure route. In the United States dairy outbreak, research published on 20 June found that the virus targets receptors abundant in cow udder tissue, where infection presents as mastitis rather than respiratory illness.

This biology affects how cases are found and how workers are protected. A study reported on 1 May detected H5N1 in submicron aerosols in milking-parlour air before clinical signs appeared in cattle. Pasteurisation destroys the virus, so commercial dairy products are not a transmission route.

Key Issues
Human cases

Cambodia drives its global case count

Between 16 February and 17 July, confirmed human H5N1 cases worldwide rose from 994 to 1,001. Cambodia accounted for four of the seven new cases, including an infant confirmed in early July.

On 1 February, a child in Bangladesh died after household poultry contact, while Cambodia recorded three human cases through the same route during 2026. This poultry-contact pathway is separate from the US dairy-cattle outbreak, so H5N1 surveillance must track distinct exposure patterns rather than treat all cases as one epidemic.

Vaccine development

H5N1 gains a Phase 3 vaccine

On 22 April, Moderna began the first Phase 3 trial of an mRNA H5N1 vaccine candidate. The trial is the furthest an mRNA vaccine for this subtype has progressed, creating efficacy and safety evidence before any pandemic strain requires rapid authorisation.

In May, CEPI committed $54.3m after US federal health authorities withdrew support. The intervention kept the trial running and made CEPI the sole funder of the most advanced active mRNA vaccine programme directed at H5N1.

Common Questions

Cambodia drives its global case count

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
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
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

H5N1 gains a Phase 3 vaccine

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
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
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
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
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
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