A: Yellow Fever vaccine is a requirement for entry to several countries in Africa and Sth America which lie in "yellow fever transmission areas"- [that is countries in which yellow fever is known to be a possible risk in some part of that country], and for re-entry into many other countries after a traveler has been in those countries or areas. What is confusing is that many countries insist on seeing the certificate of vaccination for any person before allowing entry, even though the person may not be visiting a part of the country where yellow fever actually exists. The areas and various country regulations are published by the World Health Organisation (WHO).
While there is no risk on a cruise boat, parts of Panama (not the capital), and some islands in the Caribbean are in the yellow fever transmission areas, so for cruise liner staff who may be entering many countries over time, it is better to be vaccinated and have the protection as well as an official certificate which is valid for 10 years.
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