While the west was going hysterical about Zika (which maybe caused a few thousand cases of retardation but could be used to push population control), there was a yellow fever epidemic in Angola, which even spread a few cases to China,, meaning we are lucky it didn't cause an Asian epidemic.
Yellow fever is spread by the same mosquito as Zika and Dengue, and we have an unchecked Dengue epidemic here in SEAsia.
NYTimes article on the massive vaccination effort to stop it.
they were short of the vaccine so had to get it from other third world countries, and dilute it.
and in case you didn't notice, there are a lot of Chinese working in Africa.
More than 100,000 Chinese work in Africa and many, Dr. Woodall noted, come from tropical southern China where Aedes mosquitoes already spread dengue and could spread yellow fever.
last Nov the CDC noted the vaccine shortage.
history of the vaccine here. the present vaccine has a lot of side effects (aches and pains mostly). it is a "live attenuated" virus grown in eggs. This takes times.
Since it is weak but live, it can cause problems with those who have a weak immune system (think malnutrition or HIV). In Epidemics, no one cares, but the one dollar a dosage problem is a biggie if you are a poor country who needs to vaccinate a couple million folks.
sixty thousand people die every year of yellow fever.
The good news: The shot lasts for years (you are advised one shot every ten years).
technical details for later reading LINK new vaccine in the works? LINK
more on a newer vaccine in the works, with fewer side effects
and a history of Yellow Jack and Walter Reed who figured out how it spread.