Advax-CpG55.2-adjuvanted monovalent or trivalent SARS-CoV-2 recombinant spike protein vaccine protects hamsters against heterologous infection with Beta or Delta variants

Abstract

The ongoing evolution of SARS-CoV-2 variants emphasizes the need for vaccines providing broad cross-protective immunity. This study was undertaken to assess the ability of Advax-CpG55.2 adjuvanted monovalent recombinant spike protein (Wuhan, Beta, Gamma) vaccines or a trivalent formulation to protect hamsters againstBeta or Delta virus infection. The ability of vaccines to block virus transmission to naïve co-housed animals was also assessed. In naïve hosts, the Beta variant induced higher virus loads than the Delta variant, and conversely the Delta variant caused more severe disease and was more likely to be associated with virus transmission. The trivalent vaccine formulation provided the best protection against both Beta and Delta infection and also completely prevented virus transmission. The next best performing vaccine was the original monovalent Wuhan-based vaccine. Notably, hamsters that received the monovalent Gamma spike vaccine had the highest viral loads and clinical disease of all the vaccine groups, a potential signal of antibody dependent-enhancement (ADE). These hamsters were also the most likely to transmit Delta virus to naïve recipients. In murine studies, the Gamma spike vaccine induced the highest total spike protein to RBD IgG ratio and the lowest levels of neutralizing antibody, a context that could predispose to ADE. Overall, the study results confirmed that the current SpikoGen® vaccine based on Wuhan spike protein was still able to protect against clinical disease caused by either the Beta or Delta virus variants but suggested additional protection may be obtained by combining it with extra variant spike proteins to make a multivalent formulation. This study highlights the complexity of optimizing vaccine protection against multiple SARS-CoV-2 variants and stresses the need to continue to pursue new and improved COVID-19 vaccines able to provide robust, long-lasting, and broadly cross-protective immunity against constantly evolving SARS-CoV-2 variants.

Keywords: Adjuvant; Advax; COVID-19; Coronavirus; Pandemic; SARS-Cov-2; Vaccine.

Figure 1:. Vaccine immunogenicity in mice.

Mice (n = 5 per group) were immunized IM twice 2 weeks apart with monovalent or trivalent vaccines, as indicated. Sera were collected 2 weeks after the second immunization for immunogenicity assessment. First column shows ELISA results for full spike protein binding IgG for Wuhan, Beta, Gamma, Delta and Omicron BA.2 and XBB.1 variants (mean ± SD). Second column shows ELISA results for RBD binding IgG. Third column shows calculated OD ratio of total spike IgG to RBD binding IgG. Fourth column shows spike protein pseudotyped virus neutralization titers (pVNT) (GMT ± SD). Statistical analysis was performed by one-way ANOVA with Tukey’s multiple comparison test (*; p < 0.05, **; p < 0.01, ***; p < 0.001 and ****; p < 0.0001).

Figure 2:. Vaccine immunogenicity and protection in hamsters.

Hamsters (n = 6 per group) were immunized IM twice at 2-week intervals with monovalent or trivalent vaccine, as indicated. Pre-challenge blood samples were collected 2 weeks after the second immunization for measurement of neutralizing antibody levels by PRNT₅₀ assay. (GMT ± SD) Hamsters allocated to the Beta virus challenge (n=3/group) were assayed for Beta virus PRNT₅₀ (A) and those allocated to the Delta virus challenge group for Delta PRNT₅₀ (B). Statistical analysis was performed by Kruskal-Wallis test with uncorrected Dunn’s multiple comparison test.

Figure 3.. Assessment of clinical disease.

Two weeks after the second immunization hamsters were challenged intranasally with either Beta variant (left column) or Delta variant (right column). Shown are D1-D3 maximal weight loss (mean) (A, E), D3 nasal turbinate virus load (GM) (B, F), D3 lung virus load (GM) (C, G), and D3 total lung scores (mean) (D, H). Statistical analysis was performed by Kruskal-Wallis test with uncorrected Dunn’s multiple comparison test comparing all immunized groups to the saline control group, with the exception of Delta challenge D3 lung viral load where all comparisons were made to the Gamma vaccine group.

Figure 4.. Oropharyngeal virus shedding.

Starting one day post-challenge with either Beta variant (left column) or Delta variant (right column), hamsters had oropharyngeal swabs performed daily for 3 days for assessment of virus load. Statistical analysis was performed by Kruskal-Wallis test with uncorrected Dunn’s multiple comparison test to compare all immunized groups to the saline group.

Figure 5.. Correlations of pre-challenge neutralizing antibody levels and clinical disease in challenged hamsters.

A correlation matrix was generated in Prism of Pearson correlation coefficients for PRNT₅₀ levels pre-challenge and post challenge D1–3 oropharyngeal virus load, D3 turbinate and lung viral load, D3 total lung scores and D1-D3 maximal weight loss for hamsters challenged with either Beta (A) or Delta (B) virus.

Figure 6.. Infection of naïve recipients by infected saline-control hamsters.

Two days post-challenge, each infected hamster was placed in a cage containing a single naïve animal (the recipient) for one day to assess for the ability of the infected animal to transmit the infection. Shown are virus loads in D1–4 oropharyngeal swabs, D4 nasal turbinate and D4 lung of each recipient housed with a Beta (A) or Delta (B) virus challenged saline control hamster (the donor).