The study analyzed more than 300,000 tweets that expressed an opinion about the H1N1 flu vaccine in 2009.

Twitter users who saw anti-vaccine posts in their Twitter feed tended to tweet anti-vaccine sentiments themselves, the results show. However, those who saw positive vaccine sentiments didn’t tweet positive sentiments themselves.

What’s more, positive tweets about vaccines sometimes had the opposite effect — a high number of pro-vaccine posts seemed to encourage people to tweet negatively about vaccines, said study researcher Marcel Salathé, an assistant professor of biology at Penn State University.