“The conversation around vaccines emphasizes social media more than the evidence can support,” said Brendan Nyhan, a professor of public policy at the University of Michigan who studies vaccine misinformation. “Would parents come up with some other reason not to vaccinate their kids [without misinformation on social media]? We don’t know.”¶¶That’s because the major social platforms — including Facebook, YouTube, and Pinterest — won’t share their user and engagement data with independent researchers. So researchers like Nyhan haven’t been able to study the link between exposure to fake vaccine news online and changes in people’s vaccine-seeking behavior.