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Background We have previously reported the safety and immunogenicity four weeks after two doses of the Clover COVID-19 vaccine candidate, SCB-2019, a stabilized pre-fusion form of the SARS-CoV-2 S-protein (S-trimer). We now report persistence of antibodies up to 6 months after vaccination, and cross-neutralization titers against three Variants of Concern (VoC). Methods In a phase 1 study adult (18–54 years) and elderly (55–75 years) volunteers received two vaccinations 21 days apart with 3, 9 or 30µg doses of SCB-2019 adjuvanted with CpG-1018/alum or placebo. We measured IgG antibodies against SCB-2019, ACE2-competitive-binding antibodies, and neutralizing antibodies against wild type SARS-CoV-2 (Wuhan-Hu-1) at Days 101 and 184, and neutralizing antibodies against three VoC, Alpha (B.1.1.7), Beta (B.1.351) and Gamma (P.1) in Day 36 sera. Results Titers waned from their peak at Days 36–50, but SCB-2019 IgG antibodies, ACE2-competitive binding antibodies and neutralizing antibodies against wild type SARS-CoV-2 persisted at 25–35% of their observed peak levels at Day 184. Day 36 sera also demonstrated dose-dependent increases in neutralizing titers against the three VoC. Conclusions SCB-2019 dose-dependently induced immune responses against wild-type SARS-CoV-2 which persisted up to Day 184. Neutralizing antibodies were cross-reactive against three of the most prevalent VoC.

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