Avian influenza vaccine provides protection against the disease caused by currently circulating bird flu viruses: Finland researchers
A study by the Finnish National Institute for Health and Welfare (THL) evaluated the immunity produced by the avian influenza vaccine in people in the vaccine's target group.
Avian influenza vaccinations began for those working in high-risk occupations, such as fur and poultry farm workers, veterinarians, and laboratory workers, in the summer of 2024.
They were offered a vaccine product that had been approved by the European Medicines Agency and was expected to provide protection against the avian influenza viruses circulating in Finland and Europe during 2023 and 2024.
The highly pathogenic avian influenza virus (H5N1) caused widespread outbreaks in Finland in 2023, spreading from wild birds to fur farms. During the outbreaks, several people who had contact with infected animals were exposed to the virus, which can cause serious disease in humans.
The same virus has caused widespread outbreaks in poultry around the world and in dairy cattle in the United States. In connection with the outbreaks, an exceptionally high number of infections in humans were detected during 2024.
A series of two vaccines produced antibodies that protect against bird flu
The vaccine series offered to those at increased risk of avian influenza consisted of two doses of vaccine, which were recommended to be administered at least three weeks apart. According to the THL vaccination register, by the end of January 2025, 516 people had received at least one dose of vaccine and 444 people had received two doses of vaccine.
The THL study involved 39 people who provided samples for antibody testing before vaccination and after the first and second vaccine doses. Some of the study participants (n=9) had also received avian influenza vaccines in previous years.
According to the study, the vaccine produced neutralizing antibodies that recognized the vaccine virus as well as the avian influenza viruses that caused outbreaks, which occurred on fur farms in Finland in 2023 and on dairy farms in the United States in 2024.
One dose of the vaccine produced antibody levels that are estimated to protect against disease caused by the avian influenza virus in about half of previously unvaccinated individuals, and two doses in the majority. The results suggest that the vaccine offers protection against currently circulating avian influenza viruses.
In individuals who had already received one or more avian influenza vaccines in previous years, even one dose produced high levels of neutralizing antibodies. A second dose of vaccine did not further increase the level of antibodies.
The observation indicates that previous avian influenza vaccines have produced memory cells that recognize different avian influenza viruses and that persist for a long time.
"Although the vaccine currently used in Finland is tailored to the current epidemic situation, vaccination can produce long-lasting memory cells and thus also long-term protection against various avian influenza viruses," says Merit Melin , Research Manager at THL .
The study will also assess the cell-mediated immunity produced by the avian influenza vaccine and the persistence of immunity in those vaccinated. More participants will be recruited to the study from the target groups for whom THL recommends avian influenza vaccination this spring, well in advance of the spring migration of birds from epidemic areas in Europe. The target groups for the vaccine will remain largely the same as before.
The research was carried out in collaboration with the Finnish Food Authority, the University of Turku, the Dutch Erasmus University, the HUS Diagnostic Centre and laboratory centres in the welfare areas.
Research article: Liedes et al. Inactivated Zoonotic Influenza A(H5N8) Vaccine Induces Robust Antibody Responses Against Recent Highly Pathogenic Avian Influenza Clade 2.3.4.4b A(H5N1) Viruses. The manuscript will be published as a non-peer-reviewed preprint on medRxiv in the near future.
Since 2020, highly pathogenic influenza A(H5N1) virus has been widely present in wild bird populations in Europe, especially in gulls. It has caused mass deaths in wild and farmed birds and infections in mammals. In some mammals, the disease has been severe. In 2021–2023, isolated human infections of the H5 type virus have been reported in Europe.
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WHO and ECDC (European Centre for Disease Prevention and Control) are monitoring the avian influenza situation in cooperation with national authorities. The risk of spreading infections is being minimised by timely control measures. The risk of a pandemic increases if the infection starts to spread between mammals. Influenza viruses have previously caused several pandemics in the human population.
In the summer of 2023, exceptionally large mass deaths of wild birds due to A(H5N1) virus infection have also been observed in Finland. The virus has also spread widely to fur farms, causing high morbidity and mortality in animals.
Both low- and high-pathogenicity avian influenza viruses have been found in birds in Finland, but no infections have been detected in humans. High pathogenicity means that the virus has the ability to cause more severe disease in poultry due to its genetic characteristics. High pathogenicity in birds does not indicate the ability to infect humans.