Brazil could be on the precipice of a serious yellow fever outbreak ― even though a vaccine for the disease has existed since the 1930s and is 99 percent effective.

Health authorities in the country reported at least 600 laboratory-confirmed yellow fever infections and more than 200 deaths, primarily in the Brazilian states of Minas Gerais, Espírito Santo and São Paulo, between December and April 6. Hundreds of additional cases are still being investigated.

Yellow fever can be extremely serious, according to the Mayo Clinic. Acute symptoms can include fever, headache and dizziness, while more severe symptoms, including jaundice, kidney failure and brain dysfunction, can be life-threatening.

And although yellow fever was never completely eradicated from Brazil, the current outbreak far outstrips the few dozen cases the country typically reports in rural and jungle areas each year. The current outbreak is also especially worrying because it’s close to Brazil’s densely populated urban areas, which haven’t seen a yellow fever case since 1942, according to Reuters.

Roughly half a century without a major yellow fever outbreak ― and the end of 1950s-era mosquito eradication efforts like DDT ― may have lulled Brazil into a false sense of security.

But unlike some other mosquito-transmitted diseases that threaten the Western Hemisphere ― such as dengue, chikungunya and the Zika virus ― there’s a vaccine for yellow fever. Brazil is even home to one of the world’s four yellow fever vaccine production sites, but the country’s vaccination and vaccine production efforts aren’t widespread enough to protect all of its citizens.

Because of this, Dr. Anthony Fauci of the National Institutes of Health called for public health awareness and preparedness in an April article published in the New England Journal of Medicine.

Yellow Fever Moving From Jungle To City

Yellow fever virus has three natural hosts: humans, nonhuman primates and mosquitos. So public health officials took notice when more than 1,000 howler monkeys, which are known carriers of yellow fever, dropped dead in the Brazilian forest.

“Humans have contact with mosquitos and mosquitos have contact with monkeys and humans,” Dr. Gregory Poland, head of the Mayo Clinic’s Vaccine Research Group and editor-in-chief of the journal Vaccine, told The Huffington Post.

The disease could spread rapidly through Brazil’s densely populated urban areas if an unvaccinated person is bitten by a yellow fever-carrying jungle mosquito before being bitten by an urban-dwelling Aedes aegypti mosquito.

“This proximity raises concern that, for the first time in decades, urban transmission of yellow fever will occur in Brazil,” Fauci wrote in NEJM.

“You have a tinderbox,” Poland said of Brazil’s densely populated, low-income favelas.

The disease hasn’t spread to big cities like Rio de Janeiro, São Paulo, Belo Horizonte and Vitoria, but it’s getting closer.

“Yellow fever is spreading to parts of the country where it wasn’t as prevalent,” said Dr. Anna Durbin, a professor at Johns Hopkins Bloomberg School of Public Health.

Southern areas, which haven’t traditionally had much of a yellow fever problem because of their seasonal changes and cooler climates, are seeing cases of the disease, Durbin said. It’s unclear if climate change, urbanization, deforestation or the end of mosquito eradication efforts ― or a combination of factors ― is to blame for the disease’s spread.

Now Brazil is rushing to vaccinate citizens in the south, who hadn’t previously been considered at risk for contracting yellow fever.

Not Enough Vaccine

There are upward of 170,000 severe yellow fever cases and 60,000 yellow fever deaths each year, primarily the result of localized outbreaks in Africa and Central and South Americas, according to the World Health Organization.

Major outbreaks of late ― specifically one in 2015 that began in Angola and spread to the Democratic Republic of Congo ― exhausted the world’s stockpile of yellow fever vaccine several time before the outbreak ended. In March, WHO dispatched 3.5 million doses of the world’s 6 million-dose yellow fever stockpile to Brazil.

Without sufficient stockpiles of yellow fever vaccine, Brazil is examining another emergency option: fractional dosing, or administering one-fifth of the recommended dose of yellow fever vaccine.

Fractional dosing helped end Angola’s outbreak last year. But the method has only been tested for short-term effectiveness for yellow fever in healthy adults, making it a temporary stopgap measure rather than a long-term solution to the outbreak.

Until more research is done, no one knows if the smaller dose will offer lifetime yellow fever immunity, nor whether it will be effective protection for children or people with weakened immune systems.