Outbreaks of avian influenza subtype H5N1 have been occurring since 2020, with cases reported from every continent. Some species of wild aquatic birds act as natural asymptomatic carriers of influenza A viruses, which can infect poultry, other bird species, mammals (including humans) if they come into close contact with infected feces or contaminated material, or by eating infected birds. The virus strain involved in the global outbreak is classified as H5N1 clade 2.3.4.4b, which has adapted to cause significant outbreaks in a broader range of species, mammals included.

H5N6 and H5N8 viruses with the H5-2.3.4.4b hemagglutinin (HA) gene became the dominant avian influenza viruses globally in 2018–2020 (see 2020–2023 H5N8 outbreak). In 2020, reassortment (genetic "swapping") between these H5-2.3.4.4b viruses and other strains of avian influenza led to the emergence of a H5N1 strain with a H5-2.3.4.4b gene. The virus then spread across Europe, first detected there in the autumn of 2020, before spreading to Africa and Asia. It continues to swap genes with local flu viruses as it crosses the globe.

, H5N1 avian influenza is

widespread in wild bird populations, in all continents frequently spilling over into poultry flocks, necessitating mass slaughter established in US dairy cows causing sporadic human cases, generally in people exposed to infected birds or cattle known to have infected at least 43 species of mammal

Human trials of a vaccine commenced in April 2026.

Vaccine

H5-2.3.4.4b can be prevented by vaccination in chickens. In China, the H5-Re14 (2.3.4.4b) strain used in updated vaccines since 2022 is a reasonably good match for the new virus.

In May 2024, Penn Medicine announced it had created a human avian flu vaccine on the same platform as its COVID-19 vaccine. The experimental mRNA vaccine utilizing lipid nanoparticles (LNPs) had worked to protect lab animals from severe illness and death for at least one year prior to the announcement. It was tested in mice and ferrets, and all vaccinated animals were found to survive H5N1 infections.

A 2025 study describes a recombinant subunit vaccine containing the N1 antigen from A/Michigan/45/2015 (H1N1), a virus used in seasonal flu vaccines. It provided partial protection for the A/bald eagle/Florida/W22-134-OP/2022 (H5-2.3.4.4b H5N1) virus in mice.

Timeline

2020 and prior

Genetic reassortment of several influenza A strains culminates in the emergence of a highly pathogenic H5N1 subtype bearing the clade 2.3.4.4b hemagglutinin (HA) gene.

2021 In May 2021, H5N1 was detected in wild red foxes in the Netherlands. It was later detected in December in Estonia in wild foxes. About 8,000 cranes died in Israel.

Over 1,700 great cormorants died from H5N1 in the Baltic Sea in the summers of 2021 and 2022.

2022 In January 2022, an infection in an eighty-year-old man who raises ducks in England was reported. Also in January, infections were reported from the United States in wild birds. In February, infections were reported from commercial poultry centres in the U.S., and Peru reported infections in sea lions. The virus continued to spread further, infecting additional species of mammals. In October, a mink farm in northwest Spain was affected. In December, a HPAI H5N1 subtype of clade 2.3.4.4b was found in a captive Asian black bear and in wild and captive birds in a wildlife park in France.

A human case of H5N1 was reported in the U.S. in April, "though this detection may have been the result of contamination of the nasal passages with the virus rather than actual infection." In September, Spain reported a human case; this was followed by a second case in November, in a person who worked at the same poultry farm as the first. Both were asymptomatic. In November, China reported a human case, infected due to contact with poultry. The case died from their infection.

A mass Caspian seal die-off in December 2022, with 700 infected seals found dead along the Caspian Sea coastline of Russia's Dagestan republic, worried researchers regarding the possibility that wild mammal-to-mammal spread had begun.

2023

Antarctic islands H5N1 was first detected on the islands of the Antarctic region in October 2023, via a brown skua on Bird Island, near South Georgia. Within several months, hundreds of elephant seals were found dead, as well as fur seals, kelp gulls, 77 brown skuas, 38 gentoo penguins, and 58 snowy albatrosses.

Arctic In December 2023, conservation officials confirmed that a polar bear had died of H5N1 near Alaska's northernmost city, Utqiagvik.

Brazil On 22 May 2023, Brazil declared a 180-day "animal health emergency" in response to eight cases of H5N1 found in wild birds. Although Brazil's major poultry-producing regions are in the country's south and the infections were found in Espirito Santo state and Rio de Janeiro state, Brazil, as the world's largest exporter of chicken meat, created an emergency operations center to plan for and mitigate potential further spread of H5N1.

Canada On 1 April 2023, a domestic dog in Oshawa tested positive for H5N1.

Cambodia In February 2023, Cambodia reported the death of a girl due to H5N1 infection after developing symptoms on 16 February. The girl's father also tested positive for the virus. The World Health Organization (WHO) described the situation as "worrying" and urged "heightened vigilance". Further sequencing determined that at least one of the two cases was from an older H5N1 clade, 2.3.2.1c, which had circulated as a common H5N1 strain in Cambodia for many years, rather than the more recent clade 2.3.4.4b, which had caused mass poultry deaths since 2020. This older clade had jumped to humans in the past yet hadn't previously resulted in any known human-to-human transmission.

On 1 March 2023, as Taiwan raised its travel alert for Cambodia, the WHO and the U.S. Center for Disease and Control and Prevention (CDC), in concert with Cambodian authorities, determined that both of the individuals had been infected through direct contact with poultry.

China On 4 February 2023, a 53-year-old patient was admitted to hospital with severe pneumonia in Jiangsu. Genetic sequencing revealed that the H5N1 avian influenza infected in this case belonged to clade 2.3.4.4b. She had a co-infection of H5N1 and SARS-CoV-2.

Hungary 10,000 cranes died of bird flu in Hungary.

South America In late February 2023, Argentina confirmed a case of H5N1 in industrial poultry, in the Rio Negro province. Avian product exports were suspended as a result.

In March 2023, H5N1 was detected in black-necked swan populations in Carlos Anwandter Nature Sanctuary, Chile and Uruguay. In Uruguay, the death of ten swans found in the locality of Estación Tapia was attributed to flu. Previously in Uruguay, ten hens had died because of the flu in El Monarca, Montevideo.

In late March 2023, Chile detected H5N1 in a 53-year-old man who had severe symptoms. The patient survived but had to stay on a ventilator. The virus was determined to be in the 2.3.4.4b lineage.

In September 2023, Uruguay reported upwards of 400 seals and sea lions found dead of H5N1 on the nation's Atlantic coastline and along the River Plate. Between January and October 2023, at least 24,000 South American sea lions died from H5N1 flu, with the outbreak starting on the Pacific coast of Peru, moving down the coast to Chile and then up the Atlantic coast of Argentina.

An outbreak of H5N1 killed 70% of Southern elephant seal pups born in the 2023 breeding season. In surveyed areas of Península Valdés, Argentina, seal mortality rates reached 96%. In February 2024, it was estimated that the outbreak of H5N1 in South America had killed at least 600,000 wild birds and 50,000 mammals since 2022.

2024

Antarctica H5N1 was detected in dead birds on the Antarctic mainland for the first time in February 2024. In February, scientists found H5N1 in 12 Antarctic skua seabirds carcasses on Beak Island. Additional cases have also been found at Hope Bay and on the Devil and Paulet islands. In March, scientists detected the virus in nine Adélie penguins and one Antarctic cormorant.

Australia In May 2024, H5N1 was detected for the first time in Australia after a human child who had returned to the country from India tested positive. The child was infected with the South Asian 2.3.2.1a clade of H5N1 and had severe symptoms but recovered.

H5 bird flu was detected in Australia for the first time in June 2026, after a migratory seabird in Esperance, Western Australia, tested positive for the deadly H5 strain. Authorities said there was at that time evidence of spread to poultry or mass wildlife deaths, with enhanced surveillance and biosecurity measures in place. However, the virus had spread quickly by the end of August, with over 350 confirmed events of bird flu, which included a number of mass mortality events, with around 20 species affected, including not only many species of coastal and urban birds, but also three species of mammal: a long-nosed fur seal; a common dolphin, and a fox.

there were 521 confirmed events (meaning many more individual cases), with the majority (286) found in South Australia. The most affected species is the greater crested tern. On 13 September 2026, the first confirmed case of an H5 bird flu in an endangered Australian sea lion, found at Seal Bay on Kangaroo Island, South Australia, was reported.

India On 18 April 2024, H5N1 was detected in ducks in Alappuzha district, Kerala. The District Collector decided to initiate the process of culling domestic birds within a radius from the center of the outbreak. By 9 May 2024, district officials had culled 60,232 birds in Alappuzha. Farmers were compensated ₹100 per ducklings and chicks, ₹200 per older bird, and ₹5 per egg destroyed.

Canada In November 2024, a teenager from the Vancouver region became infected with H5N1 avian influenza from an unknown source. Initially presenting with ocular symptoms, the patient later developed a serious pneumonia, indicating a novel progression of disease. The virus belongs to the 2.3.4.4b clade which has been circulating among poultry in British Columbia, related to virus carried by wild birds migrating along the Pacific flyway.

China On 18 May, Chinese authorities confirmed 275 cases of bird flu in dead Pallas's gulls and other wild birds in two counties in Qinghai province.

Southeast Asia A cluster of five human infections with H5N1 occurred in Cambodia in late January and early February; one patient died. All patients had recent contact with sick poultry. Genetic sequencing revealed that they were infected by clade 2.3.2.1c, a different lineage from the 2.3.4.4b clade that is causing global outbreaks. A person in Vietnam died of H5N1 infection around the same time, clade unknown.

In April 2024, the FAO reported that recent reassortment in the Greater Mekong Subregion has produced viruses that carry internal genes from the 2.3.4.4b lineage but the H5 gene from the older 2.3.2.1c lineage. These viruses have been implicated in human cases.

On 5 April, the Philippines reported a H5N1 outbreak on a poultry farm in Leyte, which killed 4,475 birds. Earlier in the year, the Philippines Department of Agriculture temporarily banned poultry exports from several countries including Japan, Belgium, and France.

On 6 July, it was reported that two Cambodian children became sick with H5N1 infections after handling dead chickens.

United States

The US CDC continues to report "widespread" occurrence in wild birds, "sporadic outbreaks" in poultry flocks, and "sporadic infections" in mammals as of March 2024. As of 8 March 2024, the United States Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) had recorded around 20 mammal species confirmed as being able to be infected by H5N1. Also in March 2024, H5N1 was confirmed to have infected farmed goats and cows in the USA.