I'm oil & gas. If you go to any of these places you need a Yellow Fever vaccine & certificate. They won't let you on the plane without one.

https://www.cdc.gov/yellowfever/maps/index.html

If you leave any of those those places you probably need one to transit through nearby countries to return home. It looks like this:

https://www.travelbughealth.com/sites/default/files/field/image/cert%20of%20vaccination_0.jpg

But lots of countries when you apply for work visas (not tourist) require proof of vaccination or will vaccinate you on the spot.