Vaccinations and prophylactic medications are not necessary to enter Costa Rica. Dengue and Chinkungunya fevers are the only notable diseases in this area, but they’re only occasionally reported in some communities. Taking the normal precautions to avoid mosquito bites (mosquito repellent and screens wherever necessary) is more than enough for most travelers. Also not all destinations in Costa Rica even have mosquitoes. For example, the highlands like Monteverde and San Gerardo de Dota are too cold at night for mosquitoes.
On the other hand, if you’re traveling from a country with yellow fever, you will be required to present a vaccination document. The countries required to do so when entering Costa Rica are: Angola, Benin, Bolivia, Brazil, Burkina Faso, Cameroon, Colombia, Democratic Republic of Congo, Ecuador, French Guyana, Gabon, Gambia Ghana, Guinea, Liberia, Nigeria, Peru, Sierra Leone, Sudan and Venezuela. Please make sure you check the CDC website for updated information on health and travel to Costa Rica.