Preclinical evaluation of a cross-protective ß-SARS-CoV-2 virus-like particle vaccine adjuvanted with MF59

Background: The COVID-19 pandemic led to the development of many vaccines, most notably the mRNA and recombinant adenoviral vaccines. Whilst the vaccines proved to be effective in preventing severe COVID disease, they failed to prevent infection or to control the ongoing emergence of vaccine escape variant viruses. The rapid waning of vaccine induced antibody responses has also necessitated frequent boosting. Methods: We report the findings of a recombinant ß-SARS-CoV-2 variant virus like particle (VLP) vaccine. The ß-S13EM-SARS-CoV-2 VLPs were composed of the viral spike (S), membrane (M) and envelope (E) proteins. The VLPs were produced in Vero cell factories and purified from culture supernatants using tangential flow filtration, diafiltration, anion exchange chromatography and ultrafiltration and characterised by ELISA, Western blot, and electron microscopy. The vaccine was formulated with Addavax or MF59 and tested for immunogenicity and protection in mice. Results: The ß-S13EM-SARS-CoV-2 VLPs were strongly reactive by ELISA probing with anti-S and anti-ß-RBD antibodies. Western blot showed the presence of S, M and E proteins, with the E protein present as a dimer. The ß-S13EM-SARS-CoV-2 VLPs formulated with Addavax produced strong antibody responses. T cell depletion studies confirmed that strong CD4+ responses predominated. The ß-S13EM-SARS-CoV-2/Addavax vaccine was completely protective in mice against pulmonary infection with beta SARS-CoV-2 virus. ß-S13EM-SARS-CoV-2 VLPs formulated with MF59 were similarly strongly immunogenic producing high titre anti-S and anti-ß-RBD antibodies. The vaccine was completely protective in mice against pulmonary infection with beta, delta and omicronBA.5 variant viruses. Multiplex RBD-ACE2 binding inhibition assay revealed immune sera from immunised mice produced broad neutralising antibodies (NAb) to Alpha, Delta, Beta, Gamma and Mu variants. At the highest dose of 20µg, the ß-S13EM-SARS-CoV-2 VLPs/MF59 vaccine produced NAb against against Omicron BA.1, BA.2, BA.5 and XBB1.5. Conclusion: These results demonstrate that the ß-S13EM-SARS-CoV-2 VLP formulated with Addavax or MF59 is strongly immunogenic producing broad antibody and T cell responses and is protective against infection with SARS-CoV-2 variant viruses. The vaccine offers a viable and alternative approach to supplement current COVID-19 variant vaccines.