Finnish study shows robust immune responses to H5N8 avian influenza vaccine
Finland was the first country to offer the zoonotic avian influenza A(H5N8) vaccine manufactured by Seqirus to at-risk occupational groups following the extensive clade 2.3.4.4b A(H5N1) outbreak affecting wild birds and fur farms in Finland in 2023.
A new study published in Nature Microbiology shows that the MF59-adjuvanted A(H5N8) vaccine induced strong immune responses, including both functional antibodies and memory T-cell responses, against the vaccine virus, as well as against H5 viruses that have caused recent outbreaks in Europe and the United States.
Robust immune responses after two doses
The observational phase IV study assessed antibody responses in 39 at-risk individuals and T-cell responses in a subset of 18 participants.
After two doses, the majority of previously unvaccinated individuals developed seroprotective antibody levels against the vaccine virus (A/Astrakhan/3212/2020, clade 2.3.4.4b). Seroprotection rates against the vaccine virus were 83% (95% CI 70–97%) by microneutralization assay (titer ≥20) and 97% (90–100%) by hemagglutination inhibition assay (titer ≥40).
Importantly, the antibodies also recognized heterologous clade 2.3.4.4b H5 strains, including H5N1 viruses responsible for outbreaks on Finnish fur farms and dairy cattle farms in the United States.
“These findings show that two doses of the vaccine elicit strong humoral and cellular immune responses that are expected to confer protection against currently circulating clade 2.3.4.4b H5 viruses,” the authors report.
T-cell analyses further demonstrated an approximately five-fold increase in IFN-γ producing CD4⁺ T cells after the second dose, indicating activation of cellular immunity that may contribute to broader and longer-lasting protection.
A single dose strongly boosts immunity in previously vaccinated individuals
One of the most striking findings relates to participants who had received earlier H5 vaccines, many years or more than a decade earlier. In these individuals, a single dose of the current vaccine rapidly induced high levels of neutralizing antibodies, with no significant additional boost from a closely spaced second dose.
This indicates potent immunological memory and suggests that priming at-risk populations with currently available vaccines, followed by a booster during a future epidemic, could provide rapid and robust protection even if the circulating virus differs from the vaccine strain.
Low vaccination uptake among high-risk groups
However, vaccination experiences in Finland highlight a critical gap: vaccine uptake among the targeted high-risk groups was far lower than expected, based on data from national vaccination registries. Fewer than 10% of individuals in the estimated target occupational categories received the vaccine, and not all completed the two-dose series. This underscores the need for improved communication and engagement strategies in future preparedness efforts.
Despite the strong immunogenicity, the real-world impact is limited by very low participation among those eligible for vaccination. Crucially, no fur farm workers, who were the group with the highest exposure risk during the 2023 outbreak, participated in the study despite multiple outreach efforts.
Other eligible groups included laboratory personnel, bird ringers, veterinarians, and poultry workers. Laboratory employees constituted the majority of participants in the immunogenicity study.
Several likely reasons for the low vaccine uptake
In many wellbeing services counties, responsible for organizing social and health care services, access to the vaccine was limited. Some individuals may not have been aware that they were eligible for vaccination. There may have been uncertainty about personal risk. As there was no prelicensure data on humans prior to the introduction, the limited data about the vaccine’s benefits and safety may have left people uncertain about whether to be vaccinated.
“Even if a vaccine is highly immunogenic and well matched to circulating viruses, it can only protect those who receive it,” the authors emphasize.
Improved, tailored communication strategies will be essential for future vaccination campaigns.
Implications for global avian influenza preparedness
As countries prepare for a potential escalation of H5N1 transmission globally, the Finnish experience offers important guidance. The study shows that two doses generate strong antibody and T-cell responses in previously unvaccinated individuals, while a single dose triggers rapid, high-level immunity in those primed years or even decades earlier with any H5 vaccine.
This underscores the value of priming at-risk occupational groups now during the interpandemic period, so they can be boosted quickly and effectively if the epidemiological situation worsens. Successful preparedness will also require proactive engagement of target groups to ensure access, awareness, and trust.
A strategy that includes priming at-risk individuals with currently available H5 vaccines is both sustainable and forward-looking. Should the situation deteriorate rapidly, even a non-perfectly matched booster could still provide timely and meaningful protection.
Journal
Nature Microbiology
Article Title
Influenza A(H5N8) vaccine induces humoral and cell-mediated immunity against highly pathogenic avian influenza clade 2.3.4.4b A(H5N1) viruses in at-risk individuals
Article Publication Date
5-Dec-2025
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