Background

Avian influenza viruses, which previously resided in wild birds and poultry, have recently spread to mammal populations, heightening public health concerns. The H5N1 virus, belonging to the highly pathogenic avian influenza (HPAI) clade 2.3.4.4b, is expanding its infection range through wild birds to various mammals such as cows, foxes, and seals. While human infections remain rare, the risk of emerging variants with respiratory transmissibility cannot be ruled out if mutations adapting to mammalian hosts continue to accumulate.

Existing egg-based vaccines take at least several months from strain acquisition to mass production. This is why it is difficult to respond immediately when the epidemic virus antigen changes or mutations occur simultaneously. Furthermore, the influenza virus has a rapid rate of genetic divergence, which means that vaccines targeting specific clades may not elicit sufficient immune responses against other variants, representing a limitation in their protective scope. Although messenger ribonucleic acid (mRNA) platforms capable of rapid sequence design have been identified as an alternative, the safety of general lipid nanoparticle (LNP) delivery systems remains a challenge, as they can induce acute inflammatory responses upon in vivo administration.

Key Findings

Researchers developed an mRNA vaccine encoding the hemagglutinin (HA) protein of NIID-002, a WHO-recommended candidate vaccine virus, based on a strain isolated from a wild red fox in Hokkaido, Japan (A/Ezo red fox/Hokkaido/1/2022). For the vaccine delivery system, ssPalmO, an ionizable LNP material with self-degrading properties that reduces inflammatory responses, was applied.

Experiments were conducted using a BALB/c mouse model. Following two intramuscular doses of either 1μg or 10μg of the vaccine, ELISA confirmed that NIID-002 HA antigen-specific immunoglobulin G (IgG) antibodies in the serum were strongly formed in a dose-dependent manner after the booster dose.

Lethal challenge tests to verify protective efficacy were performed with two different viral strains: A/Texas/37/2024 (TX37), belonging to the same clade 2.3.4.4b, and A/Cambodia/2311257/2023 (Cam23), from the genetically distant clade 2.3.2.1e.

In the 10μg dose group, mice showed 100% survival against both viral challenges, demonstrating cross-clade protection. In the 1 μg low-dose group, all mice survived TX37 infection without weight loss and maintained partial survival against the Cam23 challenge. Compared to the unvaccinated control group, a significant reduction in viral titers was observed in major body tissues, including the lungs, of the vaccinated mice.

Significance and Outlook

This result is noteworthy as it demonstrates in animal models that using candidate antigen genes derived from wild mammals can protect against lethal H5N1 variants from different lineages. Even if antigen sequences change, the mRNA platform allows for the re-synthesis of customized candidates within weeks. The technological advancement of integrating the degradable ssPalmO carrier to lower the burden of local and systemic inflammation—a concern with high-dose administration—is also significant.

However, it is premature to directly apply mouse experimental results to humans. Further verification of airborne transmission prevention and nasal viral suppression capabilities is required in medium-sized mammal models, such as ferrets, which more closely resemble human respiratory structures and immune responses. Assessing the duration of cross-neutralization responses as viral surface antigen mutations accelerate is also a follow-up research task for evaluating long-term immune persistence.