The COVID-19 pandemic, caused by SARS-CoV-2, necessitated the rapid development and deployment of effective vaccines. However, the emergence of SARS-CoV-2 variants of concern (VoCs), such as Alpha, Beta, Delta, and Omicron, posed significant challenges due to their increased transmissibility and ability to evade existing immunity. These variants arise through mutations during viral replication, some of which enhance receptor binding, leading to increased infectivity or immune evasion. The Beta variant, in particular, demonstrated significant immune evasion properties, showing reduced neutralization by convalescent sera and escaping several authorized monoclonal antibodies. This highlighted the need for vaccines capable of inducing broader and more robust neutralizing antibody responses against multiple variants. Novavax's prototype vaccine, NVX-CoV2373 (based on the Wuhan-Hu-1 strain), showed efficacy against the Alpha variant but reduced protection against the Beta variant. To address this limitation, the researchers developed rS-Beta, a recombinant spike protein vaccine based on the Beta variant, and evaluated its immunogenicity and protective efficacy in both mice and non-human primates.

The literature review section extensively discusses the challenges posed by emerging SARS-CoV-2 variants, particularly their ability to evade existing immunity. It cites studies showing reduced neutralization of Beta and other variants by convalescent sera and monoclonal antibodies. The review emphasizes the need for vaccines capable of inducing broader neutralizing antibody responses. It also highlights the performance of Novavax's NVX-CoV2373 vaccine, demonstrating its efficacy against some variants while also noting its limitations against others, particularly the Beta variant. The review sets the stage for the study by emphasizing the critical need for a vaccine capable of addressing the challenges posed by variant-specific immune evasion.

The study employed both in vivo and in vitro methods. For in vivo studies, BALB/c mice and olive baboons were immunized with different vaccination regimens: NVX-CoV2373 (rS-WU1), rS-Beta, a combination of both, or a heterologous prime-boost regimen. The immunogenicity of rS-Beta was assessed by measuring antibody titers (IgG) against different spike proteins (Wuhan-Hu-1, Beta, Omicron) using ELISA. The ability of sera to inhibit spike:ACE2 binding was also assessed. Neutralization assays (PRNT and microneutralization) were performed to measure the ability of the sera to neutralize different SARS-CoV-2 variants. Cell-mediated immune responses were evaluated using ELISpot and intracellular cytokine staining to assess T cell responses. Protective efficacy was evaluated by challenging vaccinated mice with Alpha and Beta variants of SARS-CoV-2 and measuring viral titers in the lungs, weight loss, and subgenomic RNA levels. Non-human primate studies (baboons) involved a primary immunization series with rS-WU1, followed by boosting with rS-Beta after approximately one year. Antibody and T cell responses were assessed in these animals. In vitro methods included characterizing the biophysical properties, structure, and function of rS-Beta using techniques such as SDS-PAGE, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and bio-layer interferometry. Statistical analysis included one-way ANOVA with post hoc tests.

The rS-Beta vaccine demonstrated robust immunogenicity in both mice and baboons. In mice, rS-Beta induced high levels of neutralizing antibodies against multiple SARS-CoV-2 variants, including Omicron BA.1. All vaccination regimens (rS-WU1, rS-Beta, combination, and heterologous prime-boost) provided significant protection against both Alpha and Beta variant challenges in mice, with undetectable viral titers in the lungs of vaccinated animals. In baboons, boosting with rS-Beta after primary immunization with rS-WU1 resulted in a strong anamnestic response, with significant increases in anti-spike IgG and neutralizing antibody titers against multiple variants. The study also showed that rS-Beta induced a Th1-skewed cellular response. In vitro analyses revealed that rS-Beta exhibited similar thermostability and hACE2 binding affinity compared to rS-WU1. A panel of variant spike protein vaccines showed that rS-Beta and rS-Beta containing vaccines demonstrated broad neutralization and inhibition of spike:ACE2 binding against a variety of variants, including Omicron. A single booster dose of rS-Beta appeared sufficient, with a second dose not providing additional benefit in the baboon model.

The findings strongly support the potential of rS-Beta as a booster vaccine to broaden protection against SARS-CoV-2 variants. The broad neutralizing capacity demonstrated against multiple variants, including Omicron, is particularly significant given the ongoing emergence of new variants. The protective efficacy observed in the mouse challenge studies, along with the robust anamnestic response in baboons, further strengthens this conclusion. The Th1-skewed cellular response suggests a contribution of cellular immunity to the overall protective effect. The study's findings have implications for vaccine strategies aimed at combating emerging SARS-CoV-2 variants and maintaining durable protection against future variants. The use of a variant-based vaccine (Beta) as a booster is shown to elicit a broader immune response compared to a homologous boosting strategy (WU1). The rapid and scalable nature of the vaccine production platform used in the study is also an important aspect, facilitating swift development and deployment of variant-specific vaccines as needed.

This study demonstrates the effectiveness of a Beta variant-based spike protein vaccine (rS-Beta) in inducing broad and robust immune responses against multiple SARS-CoV-2 variants. The vaccine's ability to elicit high titers of neutralizing antibodies and protective cellular responses, coupled with its high immunogenicity in both mice and non-human primates, highlights its potential as a valuable tool in combating the ongoing threat posed by emerging SARS-CoV-2 variants. Future research could focus on further optimizing the vaccine formulation and exploring its efficacy against additional emerging variants.

The study's limitations include the relatively small sample sizes used in the baboon studies and the potential differences between mouse and human immune responses. The mouse challenge models, while useful, may not completely recapitulate the complexity of human infection. Furthermore, the long-term durability of the immune response induced by rS-Beta remains to be fully evaluated.