The rapid development and deployment of COVID-19 vaccines was a significant achievement, yet the need for a globally accessible vaccine providing long-lasting immunity against current and future SARS-CoV-2 variants remains. As of May 2022, nearly a billion people worldwide were unvaccinated. Affordability, including cold chain requirements, and waning immunity leading to frequent booster shots are major obstacles. The emergence of SARS-CoV-2 variants capable of evading neutralizing antibodies further complicates the situation. This research addresses these challenges by developing and evaluating a novel, ferritin-based nanoparticle vaccine candidate, focusing on its immunogenicity, stability, scalability, and potential for broad-spectrum protection.

Protein nanoparticle vaccines offer advantages over isolated protein subunits due to enhanced uptake by antigen-presenting cells and multivalent antigen presentation, promoting B cell activation. Ferritin-based nanoparticle vaccines have shown promising results against SARS-CoV-2 and other viruses, with some demonstrating safety and efficacy in clinical trials. Previous research introduced SAC-Fer, a ferritin-based vaccine displaying a truncated SARS-CoV-2 spike protein. This study builds upon that foundation, improving the vaccine's immunogenicity and stability through modifications to the spike protein and the use of aluminum hydroxide as a sole adjuvant.

The researchers designed and characterized DCFH (DCFHP), an updated version of SAC-Fer with additional stabilizing proline residues. They assessed DCFH's expression levels, purity, molecular weight, stability, and epitope presentation using various techniques, including SDS-PAGE, SEC-MALS, DSF, and cryo-EM. Immunogenicity was evaluated in mice using DCFH-alum (DCFH formulated with aluminum hydroxide as the sole adjuvant). The stability of DCFH-alum was tested by storing samples at different temperatures and assessing their immunogenicity in mice. To improve production, a high-producing stable mammalian cell line expressing DCFH was developed using CHO-K1 cells. The immunogenicity of DCFH-alum was then tested in rhesus macaques using a two-dose regimen, followed by a booster dose after a varying interval. Neutralizing antibody titers against various SARS-CoV-2 variants and SARS-CoV-1 were measured. The durability of the immune response was assessed over time. T cell responses were characterized using intracellular cytokine staining assays. Various techniques were employed, including ELISA, pseudovirus neutralization assays, live virus neutralization assays, and cryo-electron microscopy.

DCFH showed improved expression levels and stability compared to SAC-Fer. DCFH-alum demonstrated robust immunogenicity in mice, even after storage at 37°C for at least two weeks. A high-producing stable CHO cell line was successfully established, enabling large-scale production. In rhesus macaques, DCFH-alum elicited strong and durable neutralizing antibody responses against multiple SARS-CoV-2 variants, including Omicron subvariants BA.4/5 and BQ.1, as well as SARS-CoV-1. A booster dose after one year generated a robust anamnestic response. The vaccine induced a balanced Th1 and Th2 CD4+ T cell response. The findings suggest that a cell-culture pseudotyped neutralizing titer of ~10-3 titers has been shown to have a correlating impact against symptomatic disease outcomes and that the observed neutralizing antibody titers in NHPs could correlate with protection against symptomatic disease in humans.

The study's findings demonstrate the potential of DCFHP-alum as a safe and effective COVID-19 vaccine. Its broad-spectrum neutralizing activity against current and past variants, along with its stability at higher temperatures and scalable production methods, address key limitations of existing vaccines. The durable immune response and strong anamnestic response following a booster suggest that DCFHP-alum could be administered annually or less frequently. The results support its potential as a booster vaccine for previously vaccinated or infected individuals, as well as a primary vaccine for children and infants. The use of aluminum hydroxide as the sole adjuvant enhances its affordability and global accessibility.

DCFHP-alum shows promise as a cost-effective, thermostable, and broadly protective COVID-19 vaccine. Its strong immunogenicity, durable response, and ability to elicit an anamnestic response after a year suggest its potential for annual or less frequent boosting and for use in pediatric populations. Further research involving clinical trials is necessary to validate these findings in humans. Future studies could investigate the optimal dosing regimen and long-term efficacy in different populations.

The study was conducted in non-human primates, and results may not fully translate to humans. The long-term durability of the immune response, beyond one year post-boost, requires further investigation. While the high-yield production method is promising, further optimization may be needed for large-scale GMP manufacturing.