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Despite increasing warnings about inaccurate information online, little is known about how social media contribute to the widespread diffusion of unverified health information. This study addresses this issue by examining the vaccine-autism controversy. By looking into a large dataset of Twitter, Reddit posts, and online news over 20 months in the US, Canada, and the UK, our time-series analysis shows that Twitter drives news agendas, and Reddit follows news agendas regarding the vaccine-autism debate. Additionally, the results show that both Twitter and Reddit are more likely to discuss the vaccine-autism link compared to online news content.

Journalists communicating risk-related uncertainty must accurately convey scientific evidence supporting particular conclusions. Scholars have explored how "balanced" coverage of opposing risk claims shapes uncertainty judgments. In situations where a preponderance of evidence points to a particular conclusion, balanced coverage reduces confidence in such a consensus and heightens uncertainty about whether a risk exists. Using the autism-vaccine controversy as a case study, we describe how journalists can cover multiple sides of an issue and provide insight into where the strength of evidence lies by focusing on "evidentiary balance." Our results suggest that evidentiary balance shapes perceived certainty that vaccines are safe, effective, and not linked to autism through the mediating role of a perception that scientists are divided about whether a link exists. Deference toward science, moreover, moderates these relationships under certain conditions. We discuss implications for journalism practice and risk communication.