“One way to stretch existing stores could be by administering a fractional dose (one-fifth of the usual dose), which has also been shown to protect against yellow fever”

One key problem is cost. In 2013, the yellow fever vaccine cost $0.82 per dose in Africa – a price that most developing countries cannot afford. A 2015 report by Médecins Sans Frontières showed that the vaccine is now almost 70 times more expensive than it was in 2001.

Making matters worse, even if countries have the money, there are serious supply constraints. Senegal’s Pasteur Institute of Dakar, one of only four facilities in the world producing yellow fever vaccines, manufactures about ten million doses per year, and the manufacturing process is extremely difficult to scale up. Moreover, the Pasteur Institute is about to close down for a five-month renovation, during which it will be unable to produce more vaccine.

Fortunately, the situation is set to improve. The Pasteur Institute is constructing a new facility about 30 kilometers from Dakar, in Diamniadio, that is expected to triple production by 2019. Another yellow fever vaccine manufacturer, Sanofi Pasteur in France, is also expanding its manufacturing capacity. (The other two manufacturers are based in Brazil and Russia.)

For now, however, supplies are tight. One way to stretch existing stores could be by administering a fractional dose (one-fifth of the usual dose), which has also been shown to protect against yellow fever.

But even that may not be enough if the virus spreads further. And, unfortunately, the high density of Aedes