John Moore/Getty Images Medical assistant Elissa Ortivez draws an MMR vaccination at the Spanish Peaks Outreach Clinic in Walsenburg, Colorado.

On Twitter, negative views of vaccination are more contagious than positive sentiments — and too many pro messages may backfire.

That’s according to Penn State University researchers who tracked about 350,000 tweets from more than 100,000 people during the 2009 H1N1 pandemic. Their study was published Thursday in the journal EPJ Data Science.

The findings have implications that extend beyond the issue of vaccinations.

“People are getting information about how to deal with their health from a variety of sources and increasingly from social media,” said the study’s lead author Marcel Salathé, assistant professor of biology and computer science at Penn State. “It’s a brave new world and we need to get in and understand it.”

The Penn State team found that exposure to negative feelings about the H1N1 vaccination spread, while positive sentiments did not catch on as easily. They also found that people exposed to more anti-vaccination talk actually became more negative, but people getting more pro-vaccination tweets did not as readily adopt a positive view.

But the most worrisome finding, said Salathé, is that intensive exposure to pro-vaccination messages on Twitter increased the likelihood that the person expressed anti-vaccination views. “This is not how we think it works,” said Salathé. “In public health you get the word out by banging the drum hard.”

From this study, Salathé can’t say if overexposure actually turned people off to vaccination. “They may have been negative from the get-go and all the positive messages made them cross a threshold and they talked back.”

While more studies are needed to explore health messages on social media, his research has important implications, he said. Public health officials can measure whether a communications strategy is working or not and change it quickly.

“They can detect patterns. If you overexpose a positive message, it may have the opposite effect,” said Salathé. “They have to be careful how they get the word out. More isn’t just better.”

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