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Unless you are going up the Amazon or trekking in the jungle yellow fever shots are not required. According to the CDC, you don't need vaccinations or medication for yellow fever or malaria if you don't plan on venturing beyond the Canal Zone. However, It would seem that the cruise line would be the final say on needing an inoculation for travel. If they say no boarding without proof of Yellow Fever inoculation, get it. I know HAL are such a line but have never heard of X demanding the yellow fever documentation.
Thanks for your reply, very helpful