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Baseline Anti-SARS-CoV-2 IgG and Protection from Symptomatic Infection: Post Hoc Analysis of the SCTV01E Phase 3
Lixin Yan1, Jiang Yi1, Dongfang Liu1
1Beijing Engineering Research Center of Protein and Antibody, Sinocelltech Ltd., No. 31 Kechuang 7th Street, BDA, Beijing 100176, China.
Vaccines
|September 27, 2025
Summary
Vaccinating individuals after natural SARS-CoV-2 infection is safe and effective, especially for those with lower antibody levels. A threshold of 338 BAU/mL for anti-SARS-CoV-2 IgG antibodies can identify individuals who benefit most from vaccination.
Area of Science:
- Immunology
- Vaccinology
- Infectious Diseases
Background:
- The necessity of COVID-19 vaccination after natural SARS-CoV-2 infection is unclear.
- Asymptomatic individuals may be vaccinated without knowing their prior infection status, impacting vaccine safety and efficacy assessments.
- Understanding the implications of vaccination in previously infected individuals is crucial for public health strategies.
Purpose of the Study:
- To analyze anti-SARS-CoV-2 IgG antibody level dynamics during a COVID-19 vaccine clinical trial.
- To evaluate the relationship between baseline IgG levels and vaccine efficacy against symptomatic SARS-CoV-2 infection.
- To assess the safety profile of the SCTV01E vaccine in individuals with varying levels of pre-existing immunity.
Main Methods:
- Analysis of daily anti-SARS-CoV-2 IgG antibody fluctuations in a Phase 3 randomized, double-blinded, placebo-controlled trial of the SCTV01E vaccine.
- Investigation of baseline IgG levels (<338, 338-1000, >1000 BAU/mL) and their correlation with protection against COVID-19 in placebo recipients.
- Assessment of vaccine efficacy and safety across different baseline IgG antibody concentration groups.
Main Results:
- Vaccination with SCTV01E demonstrated a relative protective efficacy of 69.15% against symptomatic infection in participants with baseline IgG < 338 BAU/mL.
- No additional benefit from vaccination was observed in participants with baseline IgG levels ≥ 338 BAU/mL.
- The placebo group showed significantly higher protection (93.79%) in individuals with baseline IgG ≥ 338 BAU/mL compared to those with < 338 BAU/mL; vaccine safety was comparable across IgG levels.
Conclusions:
- An anti-SARS-CoV-2 IgG antibody level of 338 BAU/mL is a suitable threshold for screening individuals in the early phase post-infection, alongside virological tests.
- Vaccinating individuals with prior SARS-CoV-2 infection is safe and does not compromise vaccine efficacy.
- The findings support tailored vaccination strategies based on pre-existing immunity levels.
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