In early 2016, shortly before the World Health Organization declared the Zika virus a global health emergency, another health crisis was quietly about to explode.

On the western coast of Africa, in the small Angolan town of Luanda, four workers who ate lunch together at a restaurant all died in December, 2015, as did the restaurant owner. Food poisoning was blamed.

Blood samples of the deceased were sent to Senegal for testing.

Weeks later, with plenty of time for a deadly virus to spread, the Pasteur Institute realized food poisoning didn’t kill these men. Rather, they died from a virus that once decimated populations of New York, in New York, Philadelphia, the Caribbean, and Europe.

And the ability of the global health community to contain it was about to be tested.

Yellow fever is, like Zika, carried by the common Aedes aegypti mosquito, along with dengue and has proven time and again to be a deadly force in the history of mankind.

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In the 1700s and 1800s, yellow fever wiped out thousands of lives in Latin America, the Caribbean, and in America, as the fledgling democracy was taking shape. Yellow fever even influenced the location of the US capitol — the Founding Fathers decided to move it to Washington, DC, in part because of the outbreak in Philadelphia in 1793.

A vaccine helped curb the virus beginning in the 1930s, but outbreaks have occurred throughout Africa since then.

While there is a cheap vaccine for yellow fever, it’s not easy to produce. The technique involves growing yellow fever vaccine inside a chicken embryo, in the egg, with the top of the shell cut off. It grows for four days, and then the embryos are plucked out of the shell, according to Science magazine. The embryos are then blended with liquid and the result is the vaccine.

The eggshell process is carried out at only a few production centers worldwide, including the Pasteur Institute in Senegal, the French pharmaceutical company Sanofi Pasteur, and two government-run factories in Russia and Brazil, according to Science.

Each dose costs about $1 to produce.

Because so few factories produce the vaccine, and because it is technically challenging and hard to mass-produce, the amount of the vaccine produced each year is limited. If demand for the yellow fever vaccine became suddenly enormous, the world’s supply may not be enough to handle it.

The World Health Organization said in June that the global emergency stockpile of yellow fever vaccine contained 6 million doses, which is “normally enough for outbreak response” in a typical year. But 2016 wasn’t a typical year.  

“In the light of the current outbreak,” the WHO said, vaccines that would normally go to other countries to immunize their populations were being redirected to Angola, and the WHO was trying to get more vaccine doses produced quickly.

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By then, yellow fever was spreading through Luanda, a city of 7 million, followed by 12 other provinces in Angola, and the next country north, Congo. Some 20 million people were suddenly at risk, according to the New York Times.

That’s six million vaccines for the 20 million lives at risk.

As the months went on, other countries around the world reported yellow fever outbreaks, further straining vaccine supplies: first Uganda, which requested 1 million vaccines, and then seven other countries (Brazil, Chad, Colombia, Ghana, Guinea, Peru and Uganda) that had outbreaks that didn’t appear to be related to the one in Angola.

The WHO debated declaring a global emergency but decided against it.

Sergio Yactayo, a yellow fever expert at the WHO, said that the organization had “seen studies” showing that one-fifth of a normal dose of the vaccine could be effective for about a year.

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And so vaccine workers diluted the 6 million doses by five times their normal strength. Together with vaccines donated by countries around the world (Brazil donated 18 million doses, while South Sudan donated its 400,000 meant for child protection), the diluting helped stretch the vaccines across some 31 million people over the next six months, according to the Times.

The WHO called the vaccination effort the “biggest emergency yellow fever vaccination campaign ever held in Africa.”

By mid-summer, there had been 6,000 cases of yellow fever suspected in Angola and Congo, with 400 people dead, but the virus had begun to slow down.

Meanwhile, health officials watched a tinderbox. The virus had been imported to China, where there was no immunity.

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If yellow fever broke out in China, 160 million doses of the vaccine diluted 10 times over would be needed to contain it, but the factories that produce the vaccine can only produce half that in a year, according to the Times.

“It did not get a foothold in Asia, but if it did, it would be a real nightmare,” Dr. Axelle Ronsse, of Doctors Without Borders, told the paper.

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Jack Woodall, a moderator at the Program for Monitoring Emerging Diseases, told Science magazine in April that an outbreak in Asia could be “impossible” to control.

“I hate being alarmist," he said. "But this is something I’m really panicking about.”

Health officials told the Times that any number of factors could have tipped the outbreak into epidemic: about 100,000 Chinese workers from tropical southern China work in Africa and could have spread the disease upon returning home; Congo’s capital had only 16 cases, but 50 would have overwhelmed its ability to contain the disease.

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In November, the World Health Organization announced the it had won “the war against yellow fever,” noting that it had been four months since the last case was reporting in Angola and the Democratic Republic of the Congo. The outbreak had a total of 7,300 suspected cases.

“This is the result of a gargantuan effort,” Dr. Margaret Lamunu, the yellow fever incident manager at the WHO, said, noting that 30 million people had been vaccinated since the beginning of the outbreak. “Can you imagine the sheer logistics of getting vaccines out to that many people?”

The WHO said that outbreaks like the one in Angola and DRC could become more frequent due to climate change, the mobility of people across borders, and the resurgence of the Aedes aegypti mosquito.

This year, while the world health community raced to find a cure or a vaccine for the new and frightening Zika virus, a familiar old foe reappeared in the yellow fever outbreak.

And while ultimately the outbreak did not tip into epidemic, the risk of a totally-preventable global health emergency became clearer than ever, demonstrating the ongoing need for resources devoted to avoiding health emergencies that are well within the global health community’s power to stop.


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