AbstractLimited data are available on the short‐ to midterm levels of antibodies to the CoronaVac vaccine and quantitative change in humoral response after homologous or heterologous booster doses. In this prospective cohort study, we evaluated the anti‐receptor‐binding domain (RBD) immunoglobulin G (IgG) levels after two doses of CoronaVac and heterologous/homologous booster administration among healthcare workers in a university hospital in Turkey. Quantitative anti‐RBD IgG antibody levels were measured at first and fourth months in 560 healthcare workers who had completed two doses of CoronaVac vaccine, and within 2 months after the third dose of CoronaVac or BNT162b2. Participants were asked to complete a questionnaire during the first blood draw. The seropositivity rate was 98.9% and 89.1%, and the median antibody level was 469.2 AU/ml and 166.5 AU/ml at first and fourth month, respectively. In the fourth month, a mean reduction of 61.4% ± 20% in antibody levels was observed in 79.8% of the participants. The presence of chronic disease (odds ratio [OR]: 1.76, 95% confidence interval [CI]: 1.15–2.69) and being in the 36–50 age group (OR: 2.11, 95% CI: 1.39–3.19) were identified as independent predictors for low antibody response. The antibody level increased 104.8‐fold (median: 17 609.4 vs. 168 AU/ml) and 8.7‐fold (median: 1237.9 vs. 141.4 AU/ml) in the participants who received BNT162b2 and CoronaVac, respectively. During the follow‐up, 25 healthcare workers (4.5%) were infected with severe acute respiratory syndrome coronavirus 2. Considering the waning immunity and circulating variants, a single booster dose of messenger RNA vaccine seems reasonable after the inactivated vaccine especially in risk groups.