Beginning Nov. 29, Costa Rica will require that tourists from 18 countries in South America and Africa have an international certificate proving they have been vaccinated for yellow fever.

Immigration Administration officials told the press yesterday this measure was declared in August, but it will not go into effect until next month.

Countries affected are Colombia, Venezuela, Brazil, Ecuador, Peru, French Guyana, Gambia, Guinea, Sierra Leone, Liberia, Ghana, Nigeria, Gabon, the Congo, Angola, Cameroon, the Sudan and Burkina Faso. These are countries where the virus is most commonly found.

Immigration Director Mario Zamora said that of these countries, Brazil is the only one that now requires a yellow fever vaccine.

Costa Rica decided to enforce this requirement because of a dengue epidemic the country has grappled with this rainy season. Yellow fever, like dengue, is transmitted by mosquito bites. Health authorities fear the dengue-transmitting aedes aegypti mosquito found in Costa Rica could also become a carrier of yellow fever.