While conspiracy theories about COVID-19 are proliferating, their impact on health-related responses during the present pandemic is not yet fully understood. We meta-analyzed correlational and longitudinal evidence from 53 studies (N = 78,625) conducted in 2020 and 2021. Conspiracy beliefs were weakly associated with more reluctance toward prevention measures both cross-sectionally and over time. They explained lower vaccination and social distancing responses but were unrelated to mask wearing and hygiene responses. Conspiracy beliefs showed an increasing association with prevention responses as the pandemic progressed and explained support for alternative treatments lacking scientific bases (e.g., chloroquine treatment, complementary medicine). Despite small and heterogenous effects, at a large scale, conspiracy beliefs are a non-negligible threat to public health.