Persistence of the Immune Responses and Cross-Neutralizing Activity With Variants of Concern Following 2 Doses of Adjuvanted SCB-2019 Coronavirus Disease 2019 Vaccine

Background. We have previously reported the safety and immunogenicity 4 weeks after 2 doses of the Clover coronavirus disease 2019 (COVID-19) vaccine candidate, SCB-2019, a stabilized prefusion form of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein (S-trimer). We now report persistence of antibodies up to 6 months after vaccination, and cross-neutralization titers against 3 variants of concern (VoCs).

Methods. In a phase 1 study, adult (18-54 years of age) and elderly (55-75 years of age) volunteers received 2 vaccinations 21 days apart with placebo or 3-, 9-, or 30-mu g. We measured immunoglobulin G (IgG) antibodies against SCB-2019, angiotensin-converting enzyme 2 (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 3 VoCs, 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 3 VoCs.

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 3 of the most prevalent VoCs.