Twitter said it permanently suspended two of the "Disinformation Dozen" accounts for repeatedly breaking its rules, required other accounts to delete some tweets and applied labels that link to credible information about vaccines and don't allow the tweets to be shared or replied to. Overall, it's removed more than 22,400 tweets for violating its COVID-19 policies.

However, spokesperson Elizabeth Busby said Twitter distinguishes between "harmful vaccine misinformation that contradicts credible public health information, which is prohibited under our policy, and negative vaccine sentiment that is a matter of opinion."

And so the "Disinformation Dozen" are still easy to find on social media.

"Tried and true" tactics

Sometimes they skirt the platforms' rules by using codes.

"Instead of saying 'vaccine,' they may, in a video, hold up the V sign with their fingers and say, 'If you're around someone who has been' — hold up V sign — 'you know, X might happen to you,' " Ahmed said.

Or they take something true and distort it, such as falsely linking a famous person's death to the fact that the celebrity got a vaccine days or weeks earlier.

NewsGuard's Gregory said a "tried and true" tactic of vaccine opponents is "grossly misrepresenting some sort of research, some sort of data to promote whatever narrative they've chosen."

Facebook said it now limits the reach of posts that could discourage people from getting vaccinated, even if the messages don't explicitly break its rules.

But the cat-and-mouse game continues.

Anti-vaccine activists claim censorship

As the social networks have cracked down, some previously prolific spreaders of vaccine misinformation have toned down their posts and have told their followers they are being censored.

Take anti-vaccine activist Robert F. Kennedy Jr., one of the "Disinformation Dozen" identified by the center, who has promoted the long discredited idea that vaccines are linked to autism. During the pandemic, he has shared baseless conspiracy theories linking 5G cellular networks to the coronavirus, and suggested, without evidence, that the death of baseball great Hank Aaron was "part of a wave of suspicious deaths" tied to vaccines.

None of that is true.

Kennedy was kicked off Instagram, which Facebook owns, in February over repeatedly sharing debunked claims.

Yet Facebook did not remove him from its namesake platform. He told NPR the company has flagged some of his posts, however, so he has become more cautious.

"I have to post, like, unicorns and kitty cat pictures on there," he said. "I don't want to give them an excuse."

He also uses it to promote his website and newsletter, where he makes claims he cannot on the social network.

Kennedy said he's never posted misinformation and accused Facebook of censorship. He said the crackdown has cost "hundreds of thousands of dollars" in donations to his organization.

A battle of persuasion

Even as the social media companies have gotten tougher recently on misinformation, researchers worry the persistence of vaccine-related hoaxes will further erode confidence among people who hesitate to get the shot.

That's especially concerning as vaccines roll out for children 12 and up.

In a survey of U.S parents, Indiana University sociologist Jessica Calarco found more than a quarter don't plan to vaccinate their kids.

"So many of these moms are turning to Facebook, are turning to Twitter, are turning to other social media platforms" for news and information, she said. "And they're saying, 'Every time I open my phone, I see something different.' "

Even some parents whose kids have had routine childhood vaccines told Calarco they're unsure about COVID-19 jabs.

Facebook this week released survey data showing vaccine acceptance among adults in the U.S. has increased by 10% since January. However, its survey also shows that the top reasons people said they don't want to get vaccinated are worries about side effects and lack of trust in the vaccines or the government — exactly the kind of fears anti-vaccination accounts promote.

The social networks said amplifying credible information from authoritative sources, such as the Centers for Disease Control and Prevention, is just as important as reducing the spread of harmful posts. Both Facebook and Twitter link to public health information in their apps and in the labels they put on misleading posts.

But they now face an uphill battle of persuading the skeptics.

Calarco said many of the parents she spoke with weigh the posts they see on social media "equally against the kinds of expert medical recommendations, expert medical information coming out of things like the CDC."

Editor's note: Facebook is among NPR's financial supporters.

Copyright NPR 2021.