What do popular YouTube™ videos say about vaccines?

Background The unregulated social network YouTubeTM has become an increasingly popular source of information on health topics such as vaccine safety. With a reach of over one billion users per month, the potential for misinformation is significant. Methods Using the keywords 'vaccine safety' and 'vaccines and children', 87 of the most widely viewed YouTubeTM videos were identified and analyzed for content, author status and view count. Results The range of view counts was 25 532 to 6 229 835, with a median of 62 075 views per video. Most videos (n = 74, 85.1%) were devoted exclusively to the topic of vaccination. The three most common sources of these YouTubeTM videos were consumers (27.6%), TV-based or Internet-based news (26.4%) and individual health professionals (25.3%). Top topics covered were autism causality (47.1% of videos), undisclosed or poorly understood risks (42.5%), adverse reactions (40.2%) and thimerosol or mercury in vaccines (36.8%). The majority of videos (65.5%) discouraged the use of vaccines. Conclusion Health professionals should be aware of the widely disseminated vaccination information available on the Internet and should appreciate its possible effect on the public.