Russian trolls and social media bots have spread false information about vaccines on Twitter, researchers say.
Tactics similar to those used in the 2016 US presidential election were deployed during an ongoing debate on social media before the election season began, according to a study from George Washington University.
Experts say the trolls were using vaccination as a "wedge issue" to promote discord through American society.
There is a prominent anti-vaccination movement in America, which links MMR injections with autism, following research by the discredited Andrew Wakefield.
David Broniatowski, an assistant professor at the School of Engineering and Applied Science, said: "The vast majority of Americans believe vaccines are safe and effective, but looking at Twitter gives the impression that there is a lot of debate.
"It turns out that many anti-vaccine tweets come from accounts whose provenance is unclear.
"These might be bots, human users or 'cyborgs' – hacked accounts that are sometimes taken over by bots.
"Although it's impossible to know exactly how many tweets were generated by bots and trolls, our findings suggest that a significant portion of the online discourse about vaccines may be generated by malicious actors with a range of hidden agendas."
Researchers looked at thousands of tweets sent from July 2014 until September 2017.
They found several accounts sending vaccine tweets belonged to the same Russian trolls believed to have interfered in the 2016 election.
They also found the bots appeared to post an equal number of pro and anti-vaccination tweets.
More than 250 tweets sent about vaccination were linked to the Internet Research Agency, a Russian government-backed company linked with attempts to infiltrate the 2016 election.
But according to the New York Times, the bots often used a different type of language to infiltrate the debate, using phrases not common in the previous dicsussions.
One bot wrote: "I don't believe in #vaccines I believe in God's will."
They also carried uncommon hashtags, like #VaccinateUS, when anti-vaxxers tend to use #Vaxxed, #b1less or #CDCWhistleblower according to the NYT.
The researchers also found the "content polluters" shared anti-vaccination messages 75% more than average Twitter users.
Mark Dredze, associate professor of computer science at Johns Hopkins, which also took part in the study, said: "These trolls seem to be using vaccination as a wedge issue, promoting discord in American society.
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"However, by playing both sides, they erode public trust in vaccination, exposing us all to the risk of infectious diseases.
"Viruses don't respect national boundaries."
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