ABSTRACT
Background: More than 12 billion doses of COVID-19 vaccine administrations and over 630 million natural infections should have developed adequate levels of herd immunity over the last three years. However, there have been many new waves of coronavirus infections. The development of safe and effective vaccines to control breakthrough SARS-CoV-2 infections remain an urgent priority. We have developed a recombinant VSV vector-based vaccine to fulfill this worldwide need. Method(s): We have used a recombinant vesicular stomatitis virus (rVSV)-based prime-boost immunization strategy to develop an effective COVID-19 recall vaccine candidate. We have constructed an attenuated recombinant VSV genome carrying the full-length Spike protein gene of SARS-CoV-2. Adding the honeybee melittin signal peptide (msp) at the N-terminus enhanced the protein expression and adding the VSV G protein transmembrane domain and the cytoplasmic tail (Gtc) at the C-terminus of the Spike protein allowed efficient incorporation of the Spike protein into pseudotype VSV. Result(s): In immunized mice, rVSV with chimeric rVSV-msp-S-Gtc induced high levels of potent neutralizing antibodies (nAbs) and CD8+ T cell responses, while the full-length Spike with Gtc proved to be the superior immunogen. More importantly, rVSV-msp-S-Gtc-vaccinated animals were completely protected from subsequent SARS-CoV-2 challenges. Furthermore, rVSV-Wuhan and rVSV-Delta vaccines, and an rVSV-Trivalent (mixed rVSV-Wuhan, -Beta and -Delta) vaccine elicited potent nAbs against live SARS-CoV-2 Wuhan (USAWA1), Beta (B.1.351), Delta (B.1.617.2) and Omicron (B.1.1.529) viruses. Heterologous boosting of rVSV-Wuhan with rVSV-Delta induced strong nAb responses against Delta and Omicron viruses, with the rVSV-Trivalent vaccine consistently inducing effective nAbs against all the SARS-CoV-2 variants tested. All rVSV-msp-S-Gtc vaccines also elicited an immunodominant Spike-specific CD8+ T cell response. Conclusion(s): rVSV vaccines targeting SARS-CoV-2 variants of concern can be considered as an effective booster vaccine in the global fight against COVID-19.