Updated beliefs and shaken confidence: evidence from vaccine hesitancy caused by experiencing “COVID arm”

BACKGROUND: Public health depends largely on people’s knowledge, beliefs, or behaviors regarding their health and medical treatments. Although works based on the health belief model have shown that public beliefs about medical treatments affect willingness to take the treatments, little is known about the effects of changes in beliefs on attitudes toward treatment. How one’s past experiences relate to one’s beliefs about a given medical treatment is worth considering. METHODS: We implemented an online panel survey in February 2021 and March 2022 in Japan before and after COVID-19 vaccines were administered to the public within the country. We exploited delayed localized hypersensitivity reactions to COVID-19 vaccines, namely, “COVID arm”, as an exogenous shock to investigate the relationship between past negative experiences and current beliefs about medical treatments or science. “COVID arm” was an unexpected side effect and thus likely caused updated beliefs about the vaccine. Out of the nonprobability sample of 15,000 respondents in the first wave in February 2021, 9,668 respondents also responded to the second wave conducted in March 2022. Outcome variables were whether experiencing “COVID arm” affected the respondents’ 1) confidence in vaccine safety, 2) willingness to take the next dose of COVID-19 vaccines, 3) acknowledgment of the importance of vaccination, and 4) confidence in science. We measured the impact of experience with “COVID arm” on changes in the probability that survey respondents would respond affirmatively to questions posed about the issues listed above. RESULTS: Experiencing “COVID arm” significantly lowered confidence in the safety of vaccination by 4.3 percentage points, which was approximately 6% of the sample mean for the first wave, and lowered the probability of taking a second dose of the COVID-19 vaccine by 1.5 percentage points. These adverse impacts were observed after conditioning background characteristics and prior confidence in vaccination. Experiencing “COVID arm” affected neither the acknowledged importance of vaccination nor confidence in science in a statistically significant way. CONCLUSIONS: An unexpected and uncomfortable shock regarding beliefs about a treatment decreases willingness to take the treatment. An appropriate public health policy should account for this effect. TRIAL REGISTRATION: The survey was preregistered with the American Economic Association’s RCT Registry (Fukai et al., 2022). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12879-023-08558-5.