“These might be bots, human users or ‘cyborgs’ – hacked accounts that are sometimes taken over by bots.”
Broniatowski said it is impossible to know how many tweets were generated, but that findings suggested a “significant portion of online discourse” about vaccines may have been generated by “malicious actors with hidden agendas”.
The study, published in the American Journal of Public Health, saw researchers examine thousands of tweets sent between July 2014 and September 2017.
They discovered several accounts – now known to belong to the same Russian trolls thought to have interfered in the US election – as well as marketing and malware bots, tweeted about vaccines and skewed online health communications.
For example, the researchers found that “content polluters” – bot accounts that distribute malware, unsolicited commercial content and disruptive materials – shared anti-vaccination messages 75% more than average Twitter users.
Meanwhile, they found that Russian trolls and more sophisticated bot accounts used a different tactic – posting equal amounts of pro- and anti-vaccination tweets.
Researchers reviewed more than 250 tweets about vaccination sent by accounts linked to the Internet Research Agency, a Russian government-backed company recently indicted by a US grand jury because of its alleged attempts to interfere in the 2016 elections.
They found the tweets used polarising language linking vaccination to controversial issues in American society, such as racial and economic disparities.
Researchers from Johns Hopkins University in Baltimore, Maryland, and the University of Maryland also took part in the study.
Mark Dredze, associate professor of computer science at Johns Hopkins, said: “These trolls seem to be using vaccination as a wedge issue, promoting discord in American society.
“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.”