Originally Posted by B747-437B

Ethiopia doesn't care and doesn't check for Yellow Fever vaccination paperwork. Their Port Health people are screening inbound passengers for Ebola and that is their main (only) focus right now.

Your next destination might check however, depending where you are headed to. Keep in mind that your passport is not usually stamped if on an Ethiopian Airlines transit STPC voucher, so there is often not any proof you entered Ethiopia at all.