Title : ELISA-based monitoring of SARS-CoV-2 IgG antibodies following infection or vaccination
Abstract:
It is broadly acknowledged that IgG antibody levels tend to decline over time, particularly around six months after the second vaccine dose. Emerging data also indicate that individuals previously infected with SARS-CoV-2 often develop a stronger and more sustained immune response post-vaccination compared to infection-naïve individuals. Nevertheless, considerable variability exists in antibody responses among individuals exposed to SARS-CoV-2 through natural infection or vaccination. Furthermore, the precise antibody thresholds required for protection, as well as the duration of immunity, remain unclear. In this study, we analyzed 3,120 serum samples from 1,248 individuals to evaluate the dynamics of SARS-CoV-2-specific IgG antibodies by monitoring anti-Spike protein antibody levels in a cohort of volunteers enrolled at the Cantacuzino National Military Medical Institute for Research and Development. Samples were collected over a 18-month period at key time points—prior to vaccination and at scheduled intervals following the first and second vaccine doses. Although the cohort included recipients of Pfizer-BioNTech, Moderna, AstraZeneca, and Janssen vaccines, the data analysis focused primarily on Pfizer-BioNTech due to its predominant use among participants. All participants provided informed consent and received no monetary or material compensation.
IgG antibody levels were measured using the Anti-SARS-CoV-2 QuantiVac ELISA (Euroimmun, Medizinische Labordiagnostika), an enzyme-linked immunosorbent assay. For detailed analysis of antibody kinetics, samples were selected based on vaccination status and prior SARS-CoV-2 infection history. Participants were stratified by time since exposure (via vaccination or infection) and by exposure type: vaccinated only, vaccinated with prior infection, or unvaccinated convalescent.
Our findings indicate that vaccinated individuals—regardless of prior infection—exhibited higher IgG antibody levels compared to unvaccinated convalescent individuals. Although antibody titers declined over time in all groups, this decrease was significantly less pronounced in individuals with a history of SARS-CoV-2 infection. These results suggest that hybrid immunity (natural infection followed by vaccination) may offer more durable protection. The study underscores the importance of longitudinal serological monitoring to better understand the persistence of humoral immunity. Additionally, demographic factors such as age and comorbidities may influence antibody kinetics and warrant further investigation.
Funding: This work was supported by the project “The importance of testing in management of the SARS-CoV-2 pandemic” (TestCoV-19) funded by MapN
