In 2015, Mark Zuckerberg weighed in on an unusually fraught issue with an uncommonly blunt statement: “Vaccination is an important and timely topic,” he wrote in a Facebook post about the book On Immunity by Eula Biss. “The science is completely clear: vaccinations work and are important for the health of everyone in our community.”

But when members of Facebook’s “community” seek information about vaccines on Facebook itself, they may be steered toward unscientific, anti-vaccination propaganda. On YouTube, a rival social media platform owned by Google, users seeking information about vaccines are similarly nudged toward anti-vaccination misinformation, much of it designed to frighten parents, even as a measles outbreak rages in the Pacific north-west.

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The Guardian found that Facebook search results for groups and pages with information about vaccines were dominated by anti-vaccination propaganda, and that YouTube’s recommendation algorithm steers viewers from fact-based medical information toward anti-vaccine misinformation.

Facebook Twitter Pinterest Facebook accepts advertising from anti-vaccination groups, boosting the performance of misinformation. Photograph: Facebook

These findingscome amid global concern over vaccine hesitancy, which the World Health Organization (WHO) named as one of its top 10 threats to global health in 2019. And they raise questions about the efficacy of Facebook and YouTube’s efforts to clamp down on harmful misinformation.

Both Facebook and YouTube have begun treating misinformation that can lead to “real-world harm” (as Facebook terms it) as a special category meriting additional scrutiny and mitigation. These policy changes followed public pressure over incidents revealing the consequences of viral misinformation, such as a spate of lynch mobs in Sri Lanka and India linked to false rumors of child abductions spreading on WhatsApp, and the harassment of victims of mass shootings in the US on YouTube.

On Friday, YouTube announced that it would reduce the amount of videos that “could misinform users in harmful ways” that it recommends to viewers. And in July, Facebook introduced a policy of deleting misinformation that was designed to provoke “violence or physical harm”.

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Neither company named anti-vaccine propaganda as a primary target of those policies, but in response to queries from the Guardian for this article, both indicated that they were moving toward taking steps to address it. A spokesman for YouTube said that some anti-vaccine videos will be treated as harmful misinformation under its new recommendation approach. And a spokeswoman for Facebook revealed that the company is exploring new options for addressing misinformation related to vaccines and other health issues.

In the meantime, the real-world harms of vaccine misinformation are well established. A recent study by the Royal Society for Public Health found that half of all parents with small children were exposed to misinformation about vaccines on social media. A public health emergency was recently declared in Washington state, where low rates of vaccination have led to a measles outbreak with 37 confirmed cases so far. According to the WHO, measles has increased by 30% globally.

Simple searches of Facebook and YouTube show how anti-vaccination propaganda can outperform fact-based information. Using a new account with no friends or likes, the Guardian used Facebook’s search bar to begin typing the word “vaccine”. Facebook’s autofill quickly began suggesting search terms that would steer a user toward anti-vaccine misinformation, such as “vaccination re-education discussion forum”, “vaccine re-education”, “vaccine truth movement” and “vaccine resistance movement”.

Facebook Twitter Pinterest Typing in a neutral search term, such as ‘vaccination’, gives results for anti-vaccine propaganda. Photograph: YouTube

Even if a user continues with a neutral search term, such as “vaccination”, the results are dominated by anti-vaccine propaganda. The top 12 groups returned by a search for “vaccination” were all anti-vaccine, led by two misinformation groups, “Stop Mandatory Vaccination” and “Vaccination Re-education Discussion Forum” with more than 140,000 members each. Facebook search results for pages were also weighted toward anti-vaccine propaganda, with eight anti-vaccine pages among the top 12.

“We are committed to accurate and useful information throughout Facebook,” said a Facebook spokeswoman, Andrea Vallone, in a statement. “We use a range of factors to inform search, including the overall popularity of a post and your Facebook connections. We remove content that violates our Community Standards, show articles that might be misleading lower, and show third-party fact-checker articles to provide people with more context. We have more to do, and will continue efforts to connect people with educational information on important topics like health.”

The company noted that anti-vaccine propaganda does not by itself violate Facebook’s rules for content.

Facebook also accepts advertising from anti-vaccination groups, boosting the performance of misinformation. The company’s political advertising archive shows how Stop Mandatory Vaccination, whose ads have included blatantly false statements such as “vaccines kill babies”, and other anti-vaccine groups use Facebook’s advertising tools to target their propaganda exclusively at women.

Facebook Twitter Pinterest On YouTube, a video by the Mayo Clinic on the measles recommends that users next watch an anti-vaccination video. Photograph: YouTube

On YouTube, autofill search suggestions also appear to default toward anti-vaccine propaganda.

And even when users find scientifically sound content about vaccines, such as a video uploaded by the Mayo Clinic about the measles, mumps and rubella vaccine (the top search result for “MMR Vaccine”), YouTube’s “up next” algorithm recommends that users next watch an anti-vaccination video.

A spokesman for YouTube said that the company does include some anti-vaccination videos in its definition of “content that could misinform users in harmful ways”, but did not clarify which. The spokesman said that the change to the recommendation algorithm will be gradual, and only affect a small number of videos.

The spokesman also noted that YouTube has begun using Wikipedia and Encyclopedia Britannica entries to provide additional information to users directly underneath videos on certain topics, including the MMR vaccine. The box defines the MMR vaccine, but a user must follow the link to find information about the fraudulent claims that have been falsely made about its supposed dangers.