Spread by Aedes aegypti mosquitoes, yellow fever is fatal in about 20 percent of cases. While a vaccine was introduced in the 1930s and successfully eliminated yellow fever epidemics in west and central Africa between 1933 and 1961, a lack of immunization has led to a resurgence. There are now an estimated 200,000 cases every year, with 90 percent in African countries.

"While highly effective—a single dose protects 99% immunized patients for 35 years or more—the yellow fever vaccine is relatively complex to produce, and there are currently only four facilities—in Brazil, Senegal, Russia, and France—equipped with the required technology.

"Orin Levine, who directs the vaccine team of the Gates Foundation points out the situation is rather telling of the general attitude towards diseases that are being kept under control through, albeit in need of continued immunization efforts. 'Nobody was looking after it until we didn’t have as many doses as we needed,' he told Quartz.

The good news, however, is we’re still in time to fix this—and with a relatively small investment. Just recently, Levine said, the Gates Foundation has invested to both ensure the lifeline of the Institut Pasteur de Dakar, the manufacturer of vaccine in Senegal, and to increase the dosage: '$1.6 million—that’s all it took to help maintain the supply,' Levine told Quartz."

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