Statistical Reasoning regarding Possible Adverse Outcomes of Vaccination

The present research examines how individuals evaluate evidence for a causal link between a vaccine and an adverse outcome, focusing on two key statistical considerations: the background incidence (or base rate) of the outcome in the absence of vaccination and the number of adverse outcomes assessed. Participants repeatedly judged the strength of evidence that a vaccine had caused an adverse outcome as information accumulated. Providing base rate information reduced perceived evidence strength and concerns about vaccine safety, particularly when the observed number of adverse events matched rather than exceeded population expectations. These effects were more pronounced among more numerate individuals. Explicitly translating base rate information into expected case counts further reduced perceived risk when the number of adverse events matched population expectations, including among less numerate participants. Conversely, explaining how monitoring many outcomes increases the likelihood of false positives had little impact on participants’ judgments. Together, these findings show how individuals’ numerical abilities interact with the presentation of statistical information to shape vaccine-related judgments, highlighting effective strategies for improving risk assessment.