Seasonal influenza represents a particularly challenging target for vaccination, due to the rapid mutation of the hemagglutinin (HA) viral coat protein. Because HA is the primary target of current influenza vaccines these mutations can reduce the effectiveness of previously acquired immunity through a phenomenon called “antigenic drift.” Another consequence of this drift is that the strains chosen for the influenza vaccine may not fully match the strains circulating that season, leading to reduced efficacy. One approach to address these challenges is to include additional targets in the seasonal influenza vaccine, such as neuraminidase (NA), the other major protein on the influenza viral coat. While some vaccine technologies do include NA, the quantity and quality is variable. In recent years, mRNA technology has proven itself as a promising approach to address emerging pandemic pathogens. One type of mRNA vaccine technology that has shown particular promise is self-amplifying mRNA (sa-mRNA). Distinct from vaccines containing standard mRNA, sa-mRNA instructs the body to replicate mRNA, amplifying the amount of protein made and providing the opportunity to include multiple proteins in a single mRNA construct. This approach has the potential to increase immunity and extend duration of protection with a reduced vaccine dose. Our lab was excited to explore the potential for a cutting edge sa-mRNA bicistronic influenza vaccine that would produce an antibody response to both HA and NA.

Our initial studies demonstrated this approach was well-suited to address the challenge of vaccines for pandemic response against avian influenza. Building on this work we utilized our bicistronic sa-mRNA platform to encode the HA and NA from four seasonal influenza strains, constructing a quadrivalent influenza vaccine, which we tested in mice and ferrets. Our findings, recently published in npj Vaccines, showed that encoding an NA component induced the production of NA-inhibiting antibodies and CD4⁺ T-cell responses in both monovalent and quadrivalent formulations. In ferret challenge studies we found the combination of HA and NA is protective at a 10-fold lower dose than either target alone, and the anti-NA response increased the breadth of neutralizing antibodies against HA-drifted strains. Our lab was extremely encouraged by these results, which demonstrate that vaccines using a next-generation bicistronic sa-mRNA platform to express both HA and NA induce neutralizing antibody responses to both viral coat proteins, as well as vaccine-specific cell-mediated immunity when formulated as a quadrivalent seasonal influenza vaccine. We hope this study spotlights the opportunity the sa-mRNA platform provides to generate immunity to multiple viral proteins with a much-reduced RNA dose, decreasing the potential for adverse events following immunization and providing a promising avenue for rapid and cost-effective vaccine production for both seasonal and pandemic influenza preparedness. Building on our laboratory’s preclinical discoveries, we are extremely proud to see this vaccine technology advance to the clinic and eagerly await the findings of a phase I study examining the safety, reactogenicity, and immunogenicity of sa-mRNA influenza vaccine in healthy adults (NCT06028347) was initiated in October 2023.

Here are some highlights from our study, and other laboratories, that support the benefits of sa-mRNA as a platform for future technologies and therapeutics:

Balanced expression of multiple genes: A bicistronic next-generation sa-mRNA construct provides balanced expression of multiple genes of interest per sa-mRNA molecule [https://www.nature.com/articles/s41541-023-00747-2], reducing the amount of mRNA and packaging lipid nanoparticles needed for multiantigen vaccines.

Broad-spectrum protection: sa-mRNA influenza vaccine candidates have produced a potent, cross-reactive immune response against pandemic and seasonal influenza strains and COVID-19 variants [https://www.nature.com/articles/s41541-023-00747-2] [https://ir.arcturusrx.com/news-releases/news-release-details/study-shows-novel-sa-mrna-vaccines-offer-robust-broad-enduring]. This suggests that sa-mRNA vaccines could offer improvements in duration and breadth of protection against new and emerging variants of infectious diseases.

Low dose range: Due to the self-amplifying nature of the vaccines, the dose range of sa-mRNA is much lower than current generation mRNA-based vaccines [https://jbiomedsci.biomedcentral.com/articles/10.1186/s12929-023-00977-5]. This could enable vaccine manufacturers to potentially develop more effective vaccines with a smaller dosage and thus lower rates of reactogenicity and greater safety.

No risk of genomic integration: mRNAs can mediate the transient expression of therapeutically functional proteins without any risk of genomic integration [https://jbiomedsci.biomedcentral.com/articles/10.1186/s12929-023-00977-5]. This makes sa-mRNA a safer alternative to other gene therapy approaches, which carry the risk of genomic integration.

Ease and efficiency of production: sa-mRNA vaccines can be rapidly synthesized using in vitro transcription technology, supporting rapid and cost-effective production [https://jbiomedsci.biomedcentral.com/articles/10.1186/s12929-023-00977-5].

As research in this space continues, we can expect to see more advancements in sa-mRNA technology and its applications in disease treatment and prevention.