COVID-19 vaccination, preventive behaviours and pro-social motivation: panel data analysis from Japan

The COVID-19 vaccine reduces infection risk; even if one contracts COVID-19, the probability of complications such as death or hospitalisation is lower. However, vaccination may prompt people to decrease preventive behaviours, such as staying home, washing hands, and wearing a mask. Therefore, if vaccinated people pursue only self-interest, the vaccine's effect may be lower than expected. However, if vaccinated people are pro-social (motivated toward benefits for the whole society), they might maintain preventive behaviours to reduce the spread of infection. We conducted 26 surveys almost once a month from March 2020 (the early stage of COVID-19) to September 2022 in Japan. By corresponding with identical individuals, we independently constructed the original panel data (N = 70,908). Based on the data, we identified the timing of the second vaccine shot and compared preventive behaviours before and after vaccination. We investigated whether second-shot vaccination correlated with changes in preventive behaviours. Furthermore, we explored whether the vaccination effect differed between the older and younger groups. We then investigated the effect of pro-social motivation on preventive behaviours. Major findings are as follows: (1) Being vaccinated led people to increase preventive behaviours, such as mask-wearing by 1.04 (95% confidence intervals [Cis]: 0.96-1.11) points, and hand hygiene by 0.34 (95% CIs: 0.30-0.38) points on a 5-point scale. (2) Vaccinated people under 65 are less likely to stay home. (3) People with pro-social motivation to be vaccinated are more likely to maintain prevention than those not so motivated; on a 5-point scale, the difference is 0.08 (95% CIs: 0.01-0.15) points for mask-wearing and 0.05 (95% CIs: 0.001-0.10) points for hand hygiene. After vaccination, the opportunity cost of staying home outweighs its benefits, and people are less inclined to stay at home. This effect is lower in older people who are at a higher risk of serious illness. The opportunity cost of wearing masks and hand hygiene is lower than that of staying home, and the benefit persists after vaccination if people are motivated to maintain these behaviours for others' well-being.