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 probably a good thing since you almost certainly don’t have one.
There is ONE EXCEPTION and that is that anyone (including travelers from the above mentioned regions) who has traveled in the last 90 days to a country where Yellow Fever is endemic must show a record of vaccination at least 7 days prior to entering Costa Rica.
If you don’t have any stamps in your passport from Africa or South America in the last 90 days then you shouldn’t have any problems.
If You Have Traveled to Africa or South America in the Past 90 Days
If you do not produce a medical record or World Health Organization international certificate of vaccination indicating that you had the vaccine against yellow fever you will be sent back to wherever your flight originated from and will not be allowed to enter Costa Rica.
We know this because it is written and because our good friends Justin and Gretchen who run Enter Costa Rica 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.