Safety and immunogenicity of SpikoGen®, an Advax-CpG55.2-adjuvanted SARS-CoV-2 spike protein vaccine: a phase 2 randomized placebo-controlled trial in both seropositive and seronegative populations

Abstract

ObjectiveWe aimed to investigate the immunogenicity and safety of SpikoGen®, a subunit COVID-19 vaccine composed of a recombinant prefusion-stabilized SARS-CoV-2 spike protein combined with the Advax-CpG55.2™ adjuvant, in seronegative and seropositive populations as primary vaccination. MethodsThis randomized, placebo-controlled, double-blind phase 2 trial was conducted on 400 participants randomized 3:1 to receive two doses of 25 μg of SpikoGen® 3 weeks apart or the placebo. The primary safety outcomes were the incidence of solicited adverse events up to 7 days after each dose and unsolicited adverse events up to 28 days after the second dose. The primary immunogenicity outcomes were seroconversion against the S1 protein and the geometric mean concentration of S1 antibodies by days 21 and 35. ResultsThe SpikoGen® vaccine was well tolerated and no serious adverse events were recorded. The most common solicited adverse events were injection site pain and fatigue, largely graded as mild and transient. By day 35 (2 weeks post second dose), the seroconversion rate against S1 was 63.55 (95% CI: 57.81–69.01) in the SpikoGen® group versus 7.23 (95% CI: 2.7–15.07) in the placebo group. The geometric mean concentration of S1 antibodies was 29.12 (95% CI: 24.32–34.87) in the SpikoGen® group versus 5.53 (95% CI: 4.39–6.97) in the placebo group. Previously infected seropositive volunteers showed a large SARS-CoV-2 humoral response after a single SpikoGen® dose. DiscussionSpikoGen® had an acceptable safety profile and induced promising humoral and cellular immune responses against SARS-CoV-2.