What Is H5N1 Bird Flu?

H5N1 is a subtype of influenza A virus classified as "highly pathogenic avian influenza" (HPAI) because it kills nearly all domesticated poultry it infects. It can occasionally jump to humans who have close contact with infected animals, with a historically reported case fatality rate of approximately 53% in confirmed cases reported to WHO since 2003 — making it one of the deadliest influenza viruses known to infect people. In 2024, H5N1 clade 2.3.4.4b spread into US dairy cattle herds in an unprecedented development, with 60+ human cases reported — mostly mild conjunctivitis and respiratory illness, unlike severe Asian cases. The virus does not currently spread efficiently between humans.

Human H5N1 infections result from close contact with infected animals or their environments:

Handling sick or dead poultry, especially at live bird markets in Asia
Contact with infected dairy cattle (mucus, raw milk, respiratory secretions) — documented in 2024 US outbreak
Exposure to heavily contaminated farm or market environments
Very rarely: exposure to a symptomatic human case (limited human-to-human clusters documented)

Risk groups: poultry workers, dairy farmers, veterinarians, wildlife workers in affected areas. General public risk is very low.

H5N1 in humans typically causes more severe illness than seasonal flu:

High fever, cough, sore throat, muscle pain
Rapid progression to pneumonia and ARDS (acute respiratory distress syndrome)
Diarrhoea and abdominal pain (atypical for flu, but common in H5N1)
2024 US dairy farm cases: predominantly mild conjunctivitis and upper respiratory symptoms

Seek immediate care if fever plus respiratory symptoms develop after animal exposure. Treatment with oseltamivir (Tamiflu) should begin within 48 hours of symptom onset.

H5N1 does not currently cause pandemics because it lacks efficient human-to-human transmission. To cause a pandemic, it would need to acquire mutations enabling sustained person-to-person spread — which experts monitor continuously. Key risk indicators:

HA receptor binding mutations (from avian α-2,3 to human α-2,6 sialic acid)
PB2 E627K — a mammalian adaptation marker found in some 2024 US isolates
Transmission chains of 3+ generations without animal contact

Current assessment: pandemic risk is elevated compared to 2023 due to cattle spread and increased human exposures, but sustained human transmission has not occurred.

Is it safe to eat poultry during an H5N1 outbreak?: Yes. Properly cooked poultry and eggs are completely safe — H5N1 is killed by cooking to internal temperature 74°C/165°F. The risk is from live animal handling, not from eating well-cooked food. Commercially processed poultry from regulated supply chains carries negligible risk.
Is pasteurized milk safe during the H5N1 cattle outbreak?: Yes. Pasteurization effectively inactivates H5N1. Multiple studies confirmed no viable H5N1 virus in commercially pasteurized milk, even from affected herds. The FDA detected H5N1 RNA (genetic fragments, not live virus) in some retail pasteurized samples — consistent with effective pasteurization kill. Avoid raw unpasteurized milk.
Is there an H5N1 vaccine for humans?: Not as a routine commercial vaccine. Governments stockpile candidate H5N1 vaccines for emergency deployment. Moderna and Pfizer are developing mRNA H5N1 vaccines in Phase 2/3 trials as of 2025. Oseltamivir (Tamiflu) is the primary treatment and prophylaxis medication and should be started within 48 hours.