Objective. The longevity of vaccine effectiveness and antibody titer after the Moderna mRNA COVID-19 vaccination booster in healthcare workers in Indonesia is not known. Materials and Methods. We performed a prospective observational study of healthcare workers at the Universitas Indonesia Hospital after Moderna mRNA COVID-19 booster vaccination. An Immunol-ogy Analyzer with Chemiluminescence Immunoassay (CLIA) test was used to examine Anti SARS-CoV-2 S-RBD levels. Antibody levels were classified into two systems (3 categories, and 2 categories). Results. There were 31 male subjects (75.6%), 33 subjects (80.5%) aged 25-39 years, 17 subjects (41.5%) with overweight BMI, 35 subjects (85.4%) without comorbidities, and 29 subjects without previous history of COVID-19 infection (70.7%) who had antibody titer >1000 AU/ml. There were 27 subjects (65.9%) who had a booster shot ≥6 months after the second vaccination with antibody titer >1000 AU/ml. In this study, there was no significant correlation between antibody titer with factors such as gender, age, BMI, comorbidities, history of COVID-19 infection and time between the 2nd vaccination and booster vaccination. Conclusion. There is no significant correlation between antibody titer with factors such as gender, age, BMI, comorbidities, history of COVID-19 infection and time between the 2nd vaccination and booster vaccination. © 2022 by the Academy of Sciences and Arts of Bosnia and Herzegovina.