Background The coronavirus disease (COVID-19) has afflicted large populations worldwide. Although vaccines aroused great expectations, their side effects on Japanese people and the antibody titer transition after vaccination are unclear. Methods The side effects of the BNT162b2 mRNA COVID-19 vaccine in participants who received vaccination at our center were investigated. Some participants were also surveyed for the antibody titer transition. Results In this study, 983 and 798 Japanese participants responded to the first and second doses, respectively. Side effects occurred in 757 (77.0%) and 715 participants (90.0%) after the first and second doses, respectively. No Grade 4 side effects occurred. The second dose had significantly more side effects than the first dose (p