There are currently no immunization requirements for travelers entering Costa Rica directly from the U.S., Europe or Canada. You do not need to carry your World Health Organization International Certificate of Vaccination – which is a good thing since unless you work for doctors without borders or the UN you probably don’t have one.
Except anyone (including travelers from the above mentioned regions) arriving from a country in South America or Africa where Yellow Fever is endemic (see list) must show a record of vaccination.
You must be vaccinated at least one week prior to entering Costa Rica. Entry is also allowed if, after visiting Africa or South America you have spent at least one week in a country with no risk of yellow fever and you have a normal body temperature when you arrive in Costa Rica.
Passing through an endemic country’s airport, port or border post on a transit visa does not require a vaccination.
If you aren’t a citizen of, or do not have any stamps in your passport from Africa or South America then you shouldn’t have any problems.
If You Have Traveled to Africa or South America
You may be sent back to wherever your flight originated and not be allowed into Costa Rica until you’ve been immunized and wait a week.
We know this because it is written and because our friends had to spend an extra week in a cheap hotel room in Ecuador laying around with the fever and other side effects of the vaccine before returning home to Arenal after their honeymoon.
Even though they were Costa Rican residents and it was obvious to Costa Rican immigration where they were headed no one mentioned that they would not be allowed back in once they landed in South America.
America: Bolivia, Brazil, Colombia, Ecuador, Peru, Venezuela, French Guyana and Trinidad and Tobago
Africa: Angola, Benin, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Democratic Republic of the Congo, Republic of the Congo, Côte d’Ivoire, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Equatorial Guinea, Kenya, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, Somalia, Sudan, South Sudan, Sao Tomé and Príncipe, Tanzania, Togo, Uganda and Zambia.