Ultra-low dose self-amplifying mRNA vaccine encoding HA and NA achieves long-term immunity against H5N1 avian influenza
Background
As the spread of avian influenza virus continues, international tension regarding the possibility of a human pandemic is rising. In particular, the highly pathogenic H5N1 virus has been reported to infect mammals in addition to birds, making the establishment of a preemptive defense barrier an urgent priority. Conventional egg-based vaccine production methods require at least 6 months from strain acquisition to mass production, making it difficult to respond agilely to sudden infectious disease crises.
While existing modified messenger RNA (mRNA) platforms have expanded the horizons of infectious disease response with rapid design speeds, limitations such as high antigen doses required for repeated vaccinations and short antibody persistence remain. This is why self-amplifying messenger RNA (sa-mRNA) technology, which contains both antigen-encoding genes and self-replicating enzymes, is attracting attention. Because it can induce long-term high immune responses even with ultra-low doses by replicating its genetic material within cells, it is considered a promising alternative to resolve vaccine supply bottlenecks.
Key Findings
Researchers at Arcturus Therapeutics (USA) released Phase 1 clinical results for ARCT-2304, a bivalent messenger RNA candidate designed to simultaneously express the two key surface proteins of the H5N1 virus: hemagglutinin (HA) and neuraminidase (NA). This randomized, observer-blind clinical trial (NCT06602531) was conducted on 212 healthy adults.
Participants were divided into three dose groups of 1.5, 5, and 12 micrograms (mcg) and received two doses of ARCT-2304 at 4-week or 8-week intervals. A placebo was administered to the control group. The primary endpoint was safety, including the incidence of adverse events, and immunogenicity measured by HA inhibition and NA enzyme-linked lectin assays 28 days after the second dose. Immunogenicity metrics were directly compared with the serum benchmarks of a previously authorized H5N1 vaccine.
The analysis results demonstrated that ARCT-2304 exhibited excellent tolerability. The reported adverse events were predominantly mild to moderate transient reactions with a dose-dependent pattern, and no serious safety issues were observed.
In the immunogenicity evaluation, robust antibody formation was confirmed in both vaccination groups. A clear difference was observed based on the vaccination interval: the 8-week interval group showed higher antibody titers and longer immune duration than the 4-week interval group. Neutralizing antibody and NA inhibitory antibody levels in the 8-week interval group remained at high levels even 6 to 7 months after the second dose. In addition to humoral immunity, the induction of cellular immunity was also confirmed. The Type 1 helper T-cell (Th1)-biased immune response, which peaked on Day 29, showed an increase proportional to the administered dose.
Significance and Outlook
ARCT-2304 elicited an immune response meeting the authorization criteria for commercially available H5N1 vaccines with an ultra-low dose of as little as 1.5 mcg. Compared to existing mRNA vaccines that required approximately 30–100 mcg per dose, this figure proves that dozens of times more people can be vaccinated with a single production batch of the vaccine. By simultaneously incorporating the NA antigen, which blocks viral release, in addition to the single HA target, it has also opened the possibility of enhancing cross-protection against variant viruses.
Long-term follow-up and large-scale subsequent validation remain challenges to be addressed. Since the Phase 1 clinical cohort size was limited to 212 participants, it is necessary to verify whether the same efficacy and safety are reproduced in large-scale Phase 2/3 clinical trials that include various age groups and patients with underlying conditions. It is also necessary to closely monitor the duration of sa-mRNA's unique intracellular replication process within the body and whether it induces rare autoimmune responses. The cost of establishing cold chain networks and the establishment of new platform product approval standards by various regulatory agencies are also cited as variables that will determine the speed of commercialization.
Avian influenza poses a persistent pandemic threat, yet traditional vaccines face production delays. Self-amplifying mRNA (sa-mRNA) technology offers potential advantages, including rapid development, dose sparing, and enhanced, durable immunity. We evaluated ARCT-2304, a bicistronic sa-mRNA vaccine encoding H5N1 hemagglutinin (HA) and neuraminidase (NA), in a Phase 1 randomized, observer-blind trial (NCT06602531). Healthy adults (N = 212) received two doses of ARCT-2304 (1.5, 5, or 12 mcg) or control/placebo on a 4- or 8-week schedule. Primary outcomes evaluated safety and reactogenicity (solicited/unsolicited/serious adverse events [AEs]) and immunogenicity (HA inhibition and NA enzyme-linked lectin assays 28 days post-second dose). Immunogenicity results were benchmarked using serum from a prior study supporting licensure of an H5N1 vaccine. ARCT-2304 was well-tolerated, displaying mostly mild-to-moderate, transient, dose-dependent solicited AEs, with no safety concerns identified. Two doses elicited robust HA and NA humoral responses. An 8-week interval yielded higher magnitude and greater durability than a 4-week schedule, with neutralizing and anti-NA antibodies remaining significantly elevated 6-7 months post-second dose. ARCT-2304 also induced dose-dependent, Th1-polarized T-cell responses peaking at Day 29. These findings support advancing sa-mRNA platforms for pandemic influenza preparedness.
This research provides a clue to fundamentally mitigate supply shortage situations in the event of an avian influenza pandemic. It becomes possible to establish policies for prioritizing the stockpiling and supply of ultra-low-dose 1.5-mcg vaccines for high-risk groups, such as poultry farm workers, slaughterhouse employees, and veterinary quarantine personnel. By producing tens of times more stockpiles within the same period without expanding existing facilities, significant cost savings can be achieved in national public health crisis response budgets. The durability of neutralizing antibodies, which lasts for 6–7 months or more, supports the design of annual vaccination cycles similar to seasonal influenza and can significantly accelerate the formation of herd immunity by reducing the burden of frequent booster shots.