Longitudinal predictors of vaccine conspiracy beliefs: findings from a five-wave panel study in Japan

The COVID-19 Pandemic not only posed a global health crisis but also intensified public divisions over vaccination, with conspiracy theories emerging as a significant obstacle to health communication. This study investigates the psychological and informational foundations of vaccine conspiracy beliefs using a five-wave longitudinal panel survey conducted annually in Japan between 2020 and 2024 (N=2,000 in 2020; N=600 in 2024). A factor analysis of an eight-item scale identified two distinct dimensions of conspiracy thinking: institutional conspiracy beliefs, which reflect distrust toward governments and pharmaceutical companies, and implausible conspiracy beliefs, which involve extreme claims such as microchip implantation. Latent growth modelling revealed that institutional conspiracy beliefs were associated with lower reliance on mass media, persistent and growing scepticism toward mainstream media, and younger age. These findings suggest that institutional conspiracy beliefs reflect enduring orientations of distrust rather than temporary reactions to pandemic-specific information. In contrast, implausible conspiracy beliefs were endorsed by only a small minority of respondents. This study makes three contributions. First, it demonstrates the need to distinguish institutional distrust-based conspiracies from fringe implausible narratives. Second, it provides longitudinal evidence clarifying how stable predispositions and changing attitudes jointly shape conspiracy beliefs. Third, it extends the literature by examining the Japanese context, adding cultural variation to a field dominated by Western-centric data.