Purpose: This study was conducted to identify factors affecting COVID-19 vaccination and associated side effects among health care workers in a small and medium-sized hospital. Methods: In May 2021, 301 workers out of a total of 670 working in a small and medium-sized hospital in Gyeonggi-do, were surveyed. The small and medium-sized hospital treats patients with COVID-19. Health care workers across different medical institutions responded with self-reported internet questionnaires. Factors influencing COVID-19 vaccination were analyzed via logistic regression. Results: Out of 301 subjects, 89.0% showed an intention to inoculate, and 85.7% were vaccinated against COVID-19. The most frequent reason for vaccination was responsibility as a medical worker. The fear of adverse reactions was the most frequent reason for non-vaccination. Adverse reactions after inoculation occurred in 70.9% of cases, and 30.6% were referred for treatment of adverse reactions. The factors ultimately influencing COVID-19 vaccination were vaccination intention, previous side effects from other vaccinations, occupation, and age. Conclusion: In order to improve the effectiveness of COVID-19 vaccination, a systematic approach is required to determine the risk factors associated with the young age of the administrative staff/personnel, subjects with a history of side effects associated with other vaccines, and health care workers who do not intend to be vaccinated. It is important to develop strategies to improve immunization. In addition, accurate and essential information regarding the side effects of vaccination is needed, along with appropriate education and publicity.