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.
"In other words, pro-vaccine messages seemed to backfire when enough of them were received," Salathé said.
The reason for this phenomenon is not clear. But it's possible that "many people had latent negative opinions about the vaccine, and when they were intensely exposed to enough positive messages, they felt the need to express their negative sentiment," Salathé said.
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