The COVID-19 vaccine reduces infection risk: even if one contracts COVID-19, the probability of complications like death or hospitalization is lower. However, vaccination may prompt people to decrease preventive behaviors, such as staying indoors, handwashing, and wearing a mask. Thereby, if vaccinated people pursue only their self-interest, the vaccine's effect may be lower than expected. However, if vaccinated people are pro-social (motivated toward benefit for the whole society), they might maintain preventive behaviors to reduce the spread of infection.