Facing the rising cases of with higher fatalities COVID-19, some countries decided to give the third dose of vaccine as a booster. As of 9 January 2022, 90.31% of health workers in Indonesia have received the third dose vaccine. This study aims to provide an evaluation of adverse events following immunization (AEFI) in a single center in Indonesia to form a basis for ensuring safety for booster administration nationally. A retrospective, cross-sectional study was conducted using an online survey. Demographic data, AEFI complaints, and factors influencing AEFIs were evaluated. In this study, there were a total of 311 subjects were gathered. The most common AEFI symptoms found at onset <24 h to 28 days were pain at the injection site, fever, shoulder pain, and headache. Most of the AEFI severity of <24 h to 28 days post-vaccination was grade 1 (reduced or uninterrupted daily activities). There was a significant correlation between AEFI and several factors, such as the history of drug allergy, exercise after vaccination, age, BMI < 25, history of symptoms after the first and second vaccinations, and history of COVID-19. There was no anaphylactic reaction in this study. Several AEFI should be considered for the third dose of COVID-19 vaccine administration.