BACKGROUND: In Indonesia, around 400 health workers died due to Covid-19 between June-July 2021, therefore the health workers need to be given further immunity. Health workers were among the first to get a booster shoot. However, they may experience side effects after vaccination. We aimed to describe side effects of Moderna vaccine as a booster in health workers. METHODS: A cross sectional study was conducted on health workers who received mRNA Covid-19 vaccine booster (Moderna) at Sulianti Saroso Infectious Disease Hospital, Indonesia and had filled the questionnaire assessing side effects form. We associated the form of the questionnaire assessing side effects from the originating source of unit in September 2021. RESULTS: A total of 101 health workers who received mRNA Covid-19 vaccine booster in Jul-Aug 2021 were included. Most of health worker experienced more than 3 side effects. The side effects were sore arm (100%); chills (72%); fatigue (57%); headache (53%) and fever (51%), other symptoms (28%). The side effects mostly happened a day of receiving a booster shot (61.4%). There was no association between age, gender, comorbid to amount of side effects (>0.05). CONCLUSION: Since the public must fulfil the immunization program during pandemic, it is the responsibility of the healthcare provider to inform about the potential side effects and benefits of a new Covid-19 vaccine.