Introduction: As an endeavor to control SARS-CoV-2 infection, the Moderna vaccine booster was given to healthcare workers to prevent reinfection and reduce the risk of complications from COVID-19. A heterologous booster vaccine is also thought to provide better protection against the current SARS-CoV-2 variants of concern. However, research that evaluates the effectiveness of the Moderna vaccine booster and the resulting SARS-CoV-2 antibody concentration is needed. Objective: To evaluate the concentration of SARS-CoV-2 antibodies after the Moderna vaccine booster and the severity of SARS-CoV-2 infection before and after the Moderna vaccine booster. Results: A total of 93 healthcare providers who received Moderna vaccine booster were included in the study. Examination of antibody concentration 3 months after the booster showed an average concentration of 10081.65 U/mL. There was an increase in antibody concentration before the booster and 3 months after, from a median of 1.7 U/mL to 9540 U/mL. Every subject showed a statistically significant increment of antibody concentration 3 months after the booster (p < 0.01). Thirty-seven (39.8%) subjects received two doses of the Sinovac vaccine and were confirmed to have COVID-19 with the Delta variant. After the booster, 26 (28%) subjects were infected with the Omicron Variant. Among the subjects who received two doses of the Sinovac vaccine and were confirmed with COVID-19, 36 (30.1%) had mild symptoms, and 1 (1.1%) was asymptomatic. Conclusions: Heterologous Moderna vaccine booster effectively increases antibody response against SARS-CoV-2 variants and shows mild symptoms of COVID-19 infection. © 2023 Fachrucha et al.