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A general rule of thumb for Africa travel, if you have been visiting or transiting through a country with a current outbreak of yellow fever or history of outbreaks of yellow fever, then you will usually need to show that you have been vaccinated against yellow fever.
Compounding the uncertainty is the fact that new outbreaks are not always publicly announced right away, but neighboring governments have been informed. So you maybe leaving a country that has an new outbreak without knowing it and then getting unexpectedly asked for proof at your next stop.
Easiest solution make sure all of your inoculations are up to date, including the "may" be required ones