Background: The decrease of immunity acquired from COVID-19 vaccines is a potential cause of breakthrough infection. Understanding the dynamics of immune responses of vaccine-induced antibodies post-vaccination is important. This study aimed to measure the level of anti-SARS-CoV-2 receptor-binding domain (RBD) total antibody in individuals at different time points upon the receipt of the second dose of CoronaVac vaccine, as well as evaluate the plausible associated factors. Methods: A cross-sectional study was conducted among CoronaVac-vaccinated residents in Banda Aceh, Indonesia. The level of anti-SARS-CoV-2 RBD total antibody was measured using Elecsys immunoassay. A set of standardized and validated questionnaires were used to assess the demographics and other associated factors. Results: Our results showed waning anti-SARS-CoV-2 RBD total antibody titres over time post-vaccination. Compared to samples of the first month post-vaccination, the antibody titres were significantly lower than those of five-months (mean 184.6 vs. 101.8 U/mL, p = 0.009) and six-months post-vaccination (mean 184.6 vs. 95.59 U/mL, p = 0.001). This suggests that the length of time post-vaccination was negatively correlated with titre of antibody. A protective level of antibody titres (threshold of 15 U/mL) was observed from all the samples vaccinated within one to three months; however, only 73.7% and 78.9% of the sera from five- and six-months possessed the protective titres, respectively. The titre of antibody was found significantly higher in sera of individuals having a regular healthy meal intake compared to those who did not (mean 136.7 vs. 110.4 U/mL, p = 0.044), including in subgroup analysis that included those five to six months post-vaccination only (mean 79.0 vs. 134.5 U/mL, p = 0.009). Conclusions: This study provides insights on the efficacy of CoronaVac vaccine in protecting individuals against SARS-CoV-2 infection over time, which may contribute to future vaccination policy management to improve and prolong protective strategy.
