Safety and immunogenicity of SARS-CoV-2 self-amplifying RNA vaccine expressing an anchored RBD: A randomized, observer-blind phase 1 study

Abstract

VLPCOV-01 is a lipid nanoparticle-encapsulated self-amplifying RNA (saRNA) vaccine that expresses a membrane-anchored receptor-binding domain (RBD) derived from the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein. A phase 1 study of VLPCOV-01 is conducted (jRCT2051210164). Participants who completed two doses of the BNT162b2 mRNA vaccine previously are randomized to receive one intramuscular vaccination of 0.3, 1.0, or 3.0 μg VLPCOV-01, 30 μg BNT162b2, or placebo. No serious adverse events have been reported. VLPCOV-01 induces robust immunoglobulin G (IgG) titers against the RBD protein that are maintained up to 26 weeks in non-elderly participants, with geometric means ranging from 5,037 (95% confidence interval [CI] 1,272-19,940) at 0.3 μg to 12,873 (95% CI 937-17,686) at 3 μg compared with 3,166 (95% CI 1,619-6,191) with 30 μg BNT162b2. Neutralizing antibody titers against all variants of SARS-CoV-2 tested are induced. VLPCOV-01 is immunogenic following low-dose administration. These findings support the potential for saRNA as a vaccine platform.

Keywords: COVID-19; RBD antigen; SARS-CoV-2; booster vaccine; immunogenicity; safety; self-amplifying RNA.

Graphical abstract

Figure 1

Participant randomization and schedule The full analysis set and safety analysis set included all participants who underwent randomization and who received one dose of VLPCOV-01, BNT162b2, or placebo. All participants who received injections completed study procedures up to 4 weeks post-study drug administration. For VLPCOV-01 administration, transition from the low-dose cohort to the next dose cohort in the same age group, and from the non-elderly cohort to the elderly cohort, was performed after the principal investigator determined that there were no medical concerns up to day 4.

Figure 2

Solicited adverse events reported up to 6 days after study drug administration (day 7) (A) The percentage of solicited local adverse events and their severity reported up to 6 days after study drug administration. (B) The percentage of systemic adverse events and their severity reported up to 6 days after study drug administration. The severity of solicited adverse events was graded as mild (gray color), moderate (turquoise color), or severe (orange color) based on criteria that are described in the Table S1. ∗No severity assessment was made for induration, only a yes or no assessment was made. Percentages shown are participants that recorded “yes.”

Figure 3

SARS-CoV-2 IgG and neutralizing antibody responses (A and B) Serum IgG titers against wild-type SARS-CoV-2 RBD protein (A) and pseudovirus neutralizing antibody titers (ID₅₀) against SARS-CoV-2 variants (B) for non-elderly (left side) and elderly (right side) participants (n = 10 per dose group, and n = 6 for the placebo group). Participants received one injection of VLPCOV-01 (0.3, 1.0, or 3.0 μg), 30 μg BNT162b2, or placebo on day 1 (week 0). Logarithmic values are reported as geometric mean titers for serum IgG and neutralizing antibody against pseudovirus. Bars indicate 95% CIs. (C) The correlation between serum neutralizing antibody titers against pseudovirus Wuhan (wild type) and IgG antibody titers against SARS-CoV-2 RBD protein following booster vaccination. Pearson’s product-moment correlation coefficient and p value were calculated following log transformation of source data (r = 0.950, p < 0.001).

Figure 4

CD4⁺ T cell responses Flow cytometric analysis was performed to analyze RBD-specific T cells. Responses to VLPCOV-01, BNT162b2, or placebo are shown as fold change from baseline (day 1) to week 4 (day 29) for each cohort. The experiment was performed once. (A) The activated CD4⁺ T cells, characterized by the expression of CD154. (B) The response in CD4⁺ Th1 cells was characterized by the expression of interleukin-2, tumor necrosis factor α, and/or interferon-γ. (C) The response in CD4⁺ Th2 cells, which was measured by expression of interleukin-4 and/or -13. (D) The response in CD4⁺ Th17 cells, characterized by the expression of interleukin-17. (E) The CD4⁺ IL-21+ cells, which were measured by expression of IL-21. (F) The correlation between IgG antibody titers against SARS-CoV-2 RBD protein and the percentage of RBD-specific CD4⁺ Th1 cell responses following vaccination with VLPCOV-01. Spearman’s rank correlation coefficient and p value were calculated following log transformation of source data (r = 0.2717, p = 0.0092). (G–K) CD8⁺ cells expressing interferon-γ, tumor necrosis factor α, CD107a, macrophage inflammatory protein 1b, or interleukin-2, respectively. (A–E and G–K) The horizontal bars indicate geometric mean value.