People are more likely to believe rumours from family and friends. There’s no algorithm mediating the experience. And when that misinformation comes in the form of forwarded texts and videos—which look the same as personal messages in WhatsApp—they’re lent another layer of legitimacy. Then you get the network compounding effect; if you’re in multiple group chats that all receive the fake news, the repetition makes them more believable still.

Of course, these are all just theories. Because of WhatsApp’s end-to-end encryption and the closed nature of its networks, it’s nearly impossible to study how misinformation moves through it. For users in countries with a history of state-sponsored violence, like Brazil, that secrecy is a feature. But it’s a bug for anyone trying to study them. “I think WhatsApp hoaxes and disinformation campaigns are a bit more pernicious [than Facebook] because their diffusion cannot be monitored,” says Pablo Ortellado, a fake news researcher and professor of public policy at the University of Sao Paulo. Misinformation on WhatsApp can only be identified when it jumps to other social media sites or bleeds into the real world.

In Brazil, it’s starting to do both. One of the videos Sacramento received from his mother-in-law is still up on YouTube, where it’s been viewed over a million times. Other stories circulated on WhatsApp are now being shared in Facebook groups with thousands of users, mostly worried mothers exchanging stories and fears. And in the streets of Rio and Sao Paulo, some people are staying away from the health workers in white coats. As of February 27, only 5.5 million people had received the shot, though it’s difficult to say how much of the slow start is due to fake news as opposed to logistical delays. A spokeswoman for the Brazilian Ministry of Health said in an email that the agency has seen an uptick in concern from residents regarding post-vaccination adverse events since the start of the year and acknowledged that the spread of false news through social media can interfere with vaccination coverage, but did not comment on its specific impact on this latest campaign.

A spokesperson for WhatsApp pointed out the public health benefits of making communications cheaper and more reliable for millions of people, while recognizing that some of them have unfortunately found ways to abuse the platform. "We encourage people to think twice before forwarding suspicious messages," the spokesperson wrote in an email. "People can always report problematic content to WhatsApp so we can be aware and help take action.”1

While the Ministry of Health has engaged in a very active pro-vaccine education operation—publishing weekly newsletters, posting on social media, and getting people on the ground at churches, temples, trade unions, and clinics—health communication researchers like Sacramento say health officials made one glaring mistake. They didn’t pay close enough attention to language.

You see, on top of all this, there’s a global yellow fever vaccine shortage going on at the moment. The vaccine is available at a limited number of clinics in the US, but it’s only used here as a travel shot. So far this year, the Centers for Disease Control and Prevention has registered no cases of the virus within US borders, though in light of the outbreak it did issue a Level 2 travel notice in January, urging all Americans traveling to the affected states in Brazil to get vaccinated first.

Because it’s endemic in the country, Brazil makes its own vaccine, and is currently ramping up production from 5 million to 10 million doses per month by June. But in the interim, authorities are administering smaller doses of what they have on hand, known as a “fractional dose.” It’s a well-demonstrated emergency maneuver, which staved off a yellow fever outbreak in the Democratic Republic of the Congo in 2016. According to the WHO, it’s “the best way to stretch vaccine supplies and protect against as many people as possible.” But a partial dose, one that’s guaranteed for only 12 months, has been met by mistrust in Brazil, where a single vaccination had always been good for a lifetime of protection.