Facebook has begun rolling out a new algorithm that directs users searching for vaccine information to the United States Centers for Disease Control (CDC) website, in the case of US-based searches, and for users elsewhere, the World Health Organization website, as a top search pick.

The move was welcomed by WHO, officials at CDC, and other health experts as an important step in combating a wave of misinformation about immunization from vaccine opponents, so-called “anti-vaxxers,” that has swept over social media. The media fog, has in turn, been blamed for alarming parents, and contributing to the recent upsurge in measles cases in the US as well as vaccine resistance elsewhere.

“We welcome Facebook’s efforts to mitigate the spread of misinformation about vaccines and connect people to sources of accurate information … social media response is an important dimension of our broader efforts to build trust and confidence in immunisation,” Dr Heidi Larson, who runs the Vaccine Confidence Project at the London School of Hygiene and Tropical Medicine, told The Guardian, which had reported in February on the fact that Facebook users were being steered through popularity algorithms to anti-vaccine sites.

Facebook announced the new policy yesterday in a company newsroom post that said, “We are working to tackle vaccine misinformation on Facebook.” The company said it would “reduce rankings” for groups and pages that spread misinformation, and it would explore ways to promote sites that “provide people more accurate information from expert organizations about vaccines at the top of results for related searches.”

WHO Director General Dr. Tedros Adhanom Ghebreyesus, said in a statement: “The World Health Organization and Facebook have been in discussions for several months to ensure people can access authoritative information on vaccines and reduce the spread of inaccuracies. Facebook will direct millions of its users to WHO’s accurate and reliable vaccine information in several languages, to ensure that vital health messages reach people who need them most.”

“Vaccine misinformation is a major threat to global health that could reverse decades of progress made in tackling preventable diseases”, the statement added, noting that many “debilitating and deadly” diseases such as diphtheria, hepatitis, polio and measles can be effectively prevented through vaccination.

Some users were quick to note the challenges inherent in the Facebook move, including for WHO, which needs to ensure that users around the world can easily get to the relevant content on the vaccine issue in different languages. “Facebook is doing the right thing and the ball is now in the court of @WHO headquarters,” tweeted one commentator complaining, “The WHO page that @Facebook redirects to is only in English and has a readability of grade 4. Has the text been pretested with vaccine-hesitant parents?”

This reporter, signing onto Facebook from Europe Thursday evening, and searching under the word “vaccine”, got to the detailed US CDC vaccine information site as a first pick and as a second pick, to the general WHO Facebook page, promoting a Walk the Talk-Health For All walk/run event planned in New York City later this month ahead of the upcoming United Nations General Assembly. A reporter testing the new Facebook algorithm from New York City also landed on the general WHO facebook page when searching for “vaccines.”

A WHO spokeswoman said she had no further details about the nature of the WHO arrangement with Facebook or how it had been reached.  However, the Facebook action followed moves earlier this year by YouTube to reduce the frequency with which users would click into anti-vaccine propaganda, as well as an announcement last week by the social media platform Pinterest that it would curb misinformation on its website.  A WHO statement last week lauded “Pinterest’s leadership in protecting public health” and called upon other social media platforms to follow its example.