What is avian influenza and why is H5N1 concerning?

Avian influenza (bird flu) is caused by influenza A viruses naturally circulating in wild aquatic birds. While most strains (LPAI) cause mild disease in birds and don't infect humans, HPAI H5N1 is highly lethal in poultry and has caused sporadic human infections since 1997 with an approximately 60% case fatality rate — the highest of any influenza strain ever documented. The H5N1 clade 2.3.4.4b that emerged around 2021 has caused the largest global bird flu epizootic in history, infecting wild birds on every continent, devastating poultry populations, and spilling into an unprecedented range of mammals — including dairy cattle in the US in 2024.

How did H5N1 get into US dairy cattle in 2024?

In early 2024, HPAI H5N1 was detected in dairy cattle herds across multiple US states — the first documented establishment of H5N1 in dairy cattle globally. Transmission appears to have occurred via migratory wild birds contaminating farm environments. The virus then spread cattle-to-cattle, likely via milking equipment and cow movement. Over 300 farm workers were infected (predominantly mild eye infections and respiratory illness from direct contact with infected cows and raw milk). This cattle reservoir is particularly concerning because it broadens the potential routes of human exposure and creates ongoing evolutionary pressure on the virus.

Can H5N1 spread human to human?

No sustained human-to-human transmission has been documented in the 25+ years of H5N1 surveillance — but this is not grounds for complacency. The virus appears to require multiple mutations to efficiently bind to the upper respiratory receptors (alpha-2,6-linked sialic acid) that would enable human-to-human spread. Laboratory studies and animal models suggest H5N1 is approximately 3-5 mutations away from pandemic potential. ECDC and CDC continuously monitor H5N1 sequences for the specific mutations (PB2 627K, PB2 701N, H226L, N186K and others) that would indicate increased mammalian adaptation.

Current risk to the general public is LOW (ECDC 2025-2026 assessment). High-risk groups: agricultural workers in direct contact with infected poultry, dairy cattle or other animals (PPE essential); veterinarians and wildlife biologists handling infected animals; laboratory workers handling H5N1 specimens; people consuming raw milk or undercooked poultry from infected farms. Immunocompromised individuals with any H5N1 exposure are at higher risk of severe disease.

Is there a vaccine for H5N1?

No H5N1 vaccine is currently recommended or approved for general use. However: national stockpiles of adjuvanted H5N1 pre-pandemic vaccines exist in the US (Audenz), EU and other countries. WHO maintains a candidate vaccine virus bank matched to current circulating strains. mRNA vaccine platforms (Moderna, BioNTech) could produce H5N1 pandemic vaccines in weeks once a pandemic strain is identified. Oseltamivir (Tamiflu) post-exposure prophylaxis is recommended for workers with unprotected exposure to infected animals.

How does GMJ News cover avian influenza?

GMJ News covers ECDC and WHO H5N1 surveillance updates, the US dairy cattle outbreak, mammalian adaptation risk, pandemic preparedness, pre-pandemic vaccine stockpiles, farm worker PPE, and One Health surveillance. Avian influenza is the defining pandemic preparedness topic of 2024-2026 — directly relevant to PHIG's health security mandate and the Georgian Medical Journal's public health coverage.

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Avian Influenza

GMJ News knowledge hub · last reviewed September 2026 · Georgian Medical Journal

Summary Statistics Glossary GMJ news FAQ Documents Organizations Research

Avian influenza — particularly the highly pathogenic H5N1 clade 2.3.4.4b strain that has caused unprecedented epizootics in wild birds, poultry and mammals since 2021 — represents the most closely monitored pandemic threat in the world today, with ECDC issuing continuous risk assessments throughout 2024-2026 and the US CDC declaring a national public health emergency after H5N1 spread into American dairy cattle herds in 2024 — the first time the virus established itself in dairy cattle globally, infecting over 300 farm workers (ECDC). With a historical human case fatality rate of approximately 60% in confirmed H5N1 infections and no sustained human-to-human transmission yet documented, avian influenza sits at the most dangerous intersection of One Health and pandemic preparedness — making it a defining topic for global public health surveillance in 2025-2026.

Key messages

#1 pandemic threat 2024-2026 — H5N1 in dairy cattle

HPAI H5N1 clade 2.3.4.4b spread into US dairy cattle in 2024 — the first time the virus established in dairy cattle globally — infecting over 300 farm workers and triggering a US public health emergency (ECDC/CDC 2024-2026 ongoing monitoring).

60% historical human CFR — no sustained human-to-human transmission (yet)

Of approximately 900+ confirmed human H5N1 cases since 2003, approximately 60% have died — the highest case fatality rate of any influenza strain. No sustained human-to-human transmission has been documented, but each spillover event carries pandemic potential.

One Health emergency — birds, mammals, humans

H5N1 has spread from wild birds to poultry, and is now spilling into an unprecedented range of mammals: dairy cattle, sea lions, seals, foxes, bears, cats, dogs. This mammalian adaptation increases the risk of mutations enabling human-to-human transmission.

Treatment: oseltamivir (Tamiflu) — must start early

Oseltamivir (Tamiflu) is the primary treatment for H5N1 in humans. It must be started within 48 hours of symptom onset for maximum effect. IV peramivir and inhaled zanamivir are alternatives. Antiviral stockpiles are maintained by WHO/national governments.

Pre-pandemic vaccines under development

Multiple pre-pandemic H5N1 vaccines are being developed and stockpiled: Moderna mRNA-1018 (H5N1), CSL Seqirus (MF59-adjuvanted), and others. WHO maintains a pre-pandemic vaccine bank. Rapid scale-up to pandemic vaccine production could take 3-6 months after a pandemic strain is identified.

Protection for farm workers — PPE is essential

Agricultural workers (poultry farmers, dairy farm workers, slaughterhouse workers) are the primary high-risk group. PPE (N95 respirator, gloves, eye protection, coveralls) dramatically reduces exposure risk. Post-exposure antiviral prophylaxis with oseltamivir reduces infection risk.

Key statistics

900+

confirmed human H5N1 cases since 2003 (WHO)

WHO 2024

~60%

historical H5N1 human case fatality rate

WHO/ECDC

300+

US farm workers infected with H5N1 from dairy cattle (2024-2025)

US CDC 2025

70+ countries

HPAI H5N1 detected in birds, mammals or humans 2024-2025

WHO/FAO/WOAH

Low

current risk to general public from H5N1 (ECDC 2026 assessment)

ECDC 2026

3-6mo

estimated time to produce pandemic H5N1 vaccine at scale

WHO/CEPI

Human H5N1 cases and deaths by year — WHO global surveillance 2003-2025

Source: WHO. H5N1 has maintained a historically high CFR across all outbreaks.

Glossary of key terms

HPAI vs LPAI
WHO/WOAHHighly pathogenic avian influenza (HPAI): causes severe systemic disease with high mortality in poultry; defined by OIE/WOAH criteria (IVPI >1.2 or sequence features). Low pathogenic avian influenza (LPAI): mild or no disease in poultry. H5N1 is HPAI. The distinction matters for trade, culling and control measures.

Clade 2.3.4.4b
WHO/FAOThe dominant H5N1 clade circulating since 2021 — responsible for the unprecedented global epizootic in wild birds, poultry and mammals. More geographically widespread and ecologically diverse than previous clades. The specific genetic lineage under WHO H5 nomenclature.

Zoonotic spillover risk
WHOSpillover events occur when H5N1 infects humans (or other mammals) from infected birds or contaminated environments. Direct contact with infected live or dead birds, contaminated environments, or — in 2024 — raw milk from infected dairy cattle are the primary exposure routes. No vaccine, no immunity in humans.

Pre-pandemic vaccine stockpile
WHO/CEPIWHO maintains a pre-pandemic H5N1 candidate vaccine virus (CVV) bank — matched to circulating strains — ready to seed manufacturing. Several countries maintain stockpiles of adjuvanted H5N1 vaccines (e.g., MF59-adjuvanted Audenz in the US). mRNA vaccine technology could dramatically speed response.

IHR and pandemic preparedness
WHO/IHRUnder the International Health Regulations (IHR), H5N1 human cases must be notified to WHO within 24 hours. ECDC coordinates European response through European Surveillance System (EpiPulse) and ECDC/EFSA quarterly avian influenza overviews. ECDC publishes rapid risk assessments with each significant new development.

One Health and H5N1
WHO/FAO/WOAHH5N1 exemplifies One Health interdependence: wild bird reservoir → domestic poultry amplification → mammalian spillover (cattle, sea mammals) → human exposure. Control requires coordinated veterinary, wildlife and human health surveillance — the core of the One Health approach.