“I’m very afraid of that vaccine. I don’t trust it,” said Muniz, who lives in Sao Paulo. “I got scared when I saw the post and thought, ‘Thank God my family hasn’t taken it yet.’ Now we’re not going to.”

Brazil is suffering one of its worst outbreaks of yellow fever, a potentially lethal mosquito-borne virus. The surge of cases comes after the country suffered an epidemic of Zika, another insect-borne virus that caused severe birth defects in hundreds of babies in 2015 and 2016.

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The yellow fever outbreak started at the end of 2016. The number of cases dropped during the Brazilian winter, when mosquitoes are less plentiful, but has surged at alarming speed since the beginning of this year in the country’s southeast. So far, the virus is being carried only by rural-dwelling mosquitoes, but cases are appearing dangerously close to three of the country’s largest metropolitan areas — Sao Paulo, Rio de Janeiro and Belo Horizonte.

Sao Paulo state, home to 45 million people, has experienced the most dramatic increase. In 2017, the state had 53 cases and 16 deaths, but in just the first six weeks of 2018, it logged 133 cases and 49 deaths. In Rio state, 27 cases and nine deaths were registered last year, and 47 cases and 21 deaths occurred in January alone this year.

The escalation has prompted a chaotic rush to vaccinate tens of millions of people through the public health system. Officials are administering partial doses of the medication, to stretch the supply, while still protecting patients for eight to 10 years. But even as the vaccination campaign expands, so does an anti-vaccine movement fueled by Internet rumors.

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“We’re seeing fake news about yellow fever spread at an alarming rate on social networks,” said Igor Sacramento, a health communication researcher at Fiocruz, one of Brazil’s largest scientific institutes.

While millions of people have camped out overnight and stood in lines that wrapped around the block to get vaccines in recent weeks in Sao Paulo and Rio, some Brazilians are opting out.

The Facebook post that alarmed Muniz was shared more than 300,000 times and was accompanied by dozens of comments from people saying they would not receive the vaccine.

But the account of the teenager dying from side effects of the vaccine was false. Officials from the town where the young woman lived confirmed that she had died but said that the cause of death was bacterial pneumonia, not the vaccine.

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The yellow-fever vaccine has been used for decades, and side effects are generally mild and include headaches and low-grade fevers. There have been reports of rare cases, however, in which people have life-threatening allergic reactions or develop diseases affecting the nervous system and internal organs. Five deaths were caused by the vaccine in Brazil last year, according to the Health Ministry.

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“One in a million people have side effects from this vaccine. That means there will be bad reactions if we’re vaccinating millions of people at the same time,” said Carla Domingues, the coordinator of the federal government’s National Immunization Program.

Facebook groups have become forums where mothers and other worried Brazilians seek information and exchange tales about yellow fever and vaccines. A group called “The Dark Side of Vaccines” has nearly 10,000 members and refers to American anti-vaccination sites such as LearnTheRisk.org and NaturalNews.com as well as translating posts from U.S.-based Facebook groups like Vaccine Resistance Movement.

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Sacramento, the researcher, said the anti-vaccine movement is growing in Brazil, but is still less articulated and political than in the United States or Europe.

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“This movement is very dangerous,” said Pedro Tauil, an epidemiologist and professor emeritus at the University of Brasilia. “We need to show people that vaccination is the best prevention — because it’s not just about individual protection, it’s also about preventing the virus from spreading to a full-blown epidemic.”

Domingues, the federal official, said the phony Internet rumors are “a new thing we have to learn to deal with and combat.” However, she said, she is not concerned about fake news affecting the number of people getting vaccinated, citing the long lines that have formed during the vaccination campaign.

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The reach of social-media posts in this country of over 200 million can be staggering. A Facebook account listed as belonging to a Christian nonprofit organization posted a video, which was viewed 4.5 million times, showing a tearful woman detailing what she called her son’s near-fatal allergic reaction to the yellow fever vaccine.

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“We need to understand if all these people are dying because they actually had yellow fever, or if it’s because of a reaction” to the vaccine, she says. The text accompanying the post says, “Vaccines kill . . . share this so that people become aware that their biggest enemy is not an animal, but actually is the state itself, driven by powerful, hidden forces.”

WhatsApp, the country’s most popular messaging app, has also been used to convey fake information. In January, an audio message circulated on WhatsApp with an unidentified woman claiming to be a doctor at a well-known laboratory warning that the yellow-fever vaccine is dangerous.

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A rumor has also made the rounds on WhatsApp claiming that drinking a blend of fruits and vegetables every day immunizes people against yellow fever. “The vaccine is not safe. Share this recipe so that more people will be immunized from yellow fever,” the message says.

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