Baseline Anti-SARS-CoV-2 IgG and Protection from Symptomatic Infection: Post Hoc Analysis of the SCTV01E Phase 3 Randomized Trial

<b>Background:</b> Following SARS-CoV-2 infection, the necessity of vaccination after natural infection remains uncertain. However, many asymptomatic individuals who test negative virologically may nevertheless receive vaccination without being aware of their prior infection. Investigating the implications for vaccine safety and efficacy is crucial. <b>Methods:</b> We analyzed the daily fluctuations in anti-SARS-CoV-2 IgG antibody levels during the enrollment period of a phase 3 randomized, double-blinded, placebo-controlled clinical trial of a tetravalent COVID-19 protein vaccine, SCTV01E. Additionally, we investigated the relationship between baseline IgG levels and their protection against COVID-19 in participants who received placebo. <b>Results:</b> The daily enrolled participants with different baseline IgG levels (<338 BAU/mL, 338-1000 BAU/mL, >1000 BAU/mL) showed dynamic changes with the enrollment date. Among participants with baseline IgG levels < 338 BAU/mL, vaccination conferred a relative protective efficacy of 69.15% (95% CI: 51.14-80.52%) against symptomatic SARS-CoV-2 infection compared with the control group. Conversely, in those with higher baseline IgG levels (≥338 BAU/mL), vaccination did not confer additional benefit. In the placebo group, the relative protection in participants with baseline IgG levels ≥ 338 BAU/mL was 93.79% (87.60%, 96.89%) compared to that of those with baseline IgG levels < 338 BAU/mL. The safety profile of SCTV01E in participants with baseline IgG ≥ 338 BAU/mL was comparable to that in participants with <338 BAU/mL, with favorable safety profiles. <b>Conclusions:</b> During the SCTV01E phase 3 clinical trial, an anti-SARS-CoV-2 IgG antibody IgG level of 338 BAU/mL was suitable for screening individuals in the early phase post-infection alongside virological tests. Vaccinating the infected population was safe and did not compromise efficacy. Clinical Trial: NCT05308576.