Publication date: Feb 14, 2025

Vaccine-induced neutralizing antibodies (NAbs) are key for COVID-19 protective-immunity. As the efficacy of SARS-CoV-2 vaccines declines over time and variants of the virus continue to emerge, the need for booster doses of vaccine remains on the agenda. The aim of this study was to assess NAbs dynamics and its correlation with anti-RBD IgG levels during the nine-month follow-up period after primary-CoronaVac vaccination and booster vaccinations to evaluate vaccination strategies. This prospective longitudinal observational study followed 226 healthcare workers who received primary (two doses CoronaVac) and booster (CoronaVac or BNT162b2) immunization. Serum samples were collected at four different time points, two after primary vaccination and two after booster. Anti-RBD IgG antibody levels were assessed with the SARS CoV-2 IgG-II-QUANT kit (Abbott, USA) and neutralizing antibody levels were determined with the ACE2-RBD-Neutralization-Assay (Dia-Pro, Italy) using a surrogate virus neutralization method. Factors affecting antibody response were analyzed. Statistical analysis was performed with IBM-SPSS-22. 0. One month after the second dose of CoronaVac, 79. 2% of participants had NAb, but this had decreased to 49. 7% by the fourth month and was influenced by smoking, BMI and chronic diseases. Boosters, regardless of type, significantly raised NAb levels. Heterologous vaccination yielded higher NAb and anti-RBD IgG responses. Both single or double-BNT162b2 boosters resulted in similar NAb responses. There was a strong correlation between anti-RBD IgG and NAb levels following CoronaVac vaccination, leading to the identification of predictive IgG threshold for the presence of NAb. The type of booster influenced the correlation strength and threshold-value. NAbs levels decreased rapidly after primary CoronaVac vaccination. Boosters significantly increased levels while the heterologous vaccine combination induced a greater response. Anti-RBD IgG levels were able to predict the NAb response, however the correlation varied by the vaccine type, NAb response strength and the time since vaccination.

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