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.