Which countries need Yellow Fever Vaccination? How many doses are needed? Which countries need Yellow Fever Vaccination? How many doses are needed?

If you are travelling to many countries in Africa or South America you need to Yellow Fever Vaccination certificate with a yellow WHO International certificate of vaccination booklet. Is this important even if you are only transiting and not stopping in the country too? Do we still need a booster dose for the Yellow Fever vaccine? Or is a single dose sufficient for life? कौनसे देश हैं जहाँ जाने के लिए आपको येलो फीवर का टीका लगाना ज़रूरी है? इसकी बूस्टर डोस की ज़रुरत अब भी है क्या? https://youtu.be/lHZ6k1CSRvc Charak Clinics