This is a study of drug effectiveness for 2 treatments of vivax malaria, which is one of the two main types of malaria in Viet Nam. There are two important drugs used in Viet Nam for treating vivax malaria, Chloroquine and Artemisinin. Sometimes, when medicines are used for many years they become less effective at treating a disease, especially when they are not used at adequate doses according to national guidelines or when counterfeit drugs are available in the market. The purpose of this study is to check that Chloroquine and Artemisinin, are still effective for patients in Viet Nam.