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.

Participants in this study will be treated with either Dihydroartemisinin-Piperaquine (DHA-PPQ) or Chloroquine (CQ) for 3 days. Both drugs are recommended by the national guidelines to treat vivax malaria. The investigators would like to know if both of these treatments are equally effective so half of the patients in the study will be treated with DHA-PPQ and the other half will be treated with CQ. This way the investigators can compare the drugs to find out if one is better than the other.

Participants will be followed for 3 days in hospital, then regularly by follow-up visits until the 63rd day. Tests will be done to determine the amount of drug and malaria parasites in the participant's body and how the blood cells react to the malaria. The parasite will be tested to determine what type it is and how it reacts to the treatment.

The results of the study will be used to inform malaria treatment guidelines in Viet Nam.

Inclusion Criteria:

• Age > 3 years;
• Mono-infection with P. vivax, parasitemia > 250/µl asexual forms for in vivo and >8000 asexual parasites/µl blood for in vitro testing;
• Presence of axillary or tympanic temperature ≥ 37.5 °C or history of fever during the past 24 h;
• Ability to swallow oral medication;
• Ability and willingness to comply with the study protocol for the duration of the study and to comply with the study visit schedule;
• Informed consent/assent

Exclusion Criteria:

• Presence of general danger signs or severe malaria according to the definitions of WHO (2000);
• Mixed infection with P.falciparum and P.vivax of other plasmodium species;
• Presence of febrile conditions due to diseases other than malaria (e.g. measles, acute lower respiratory tract infection, severe diarrhoea with dehydration) or other known underlying chronic or severe diseases (e.g. cardiac, renal and hepatic diseases, HIV/AIDS);
• Regular medication, which may interfere with antimalarial pharmacokinetics;
• Received antimalarial drugs in the previous 48 hours;
• History of hypersensitivity reactions or contraindications to any of the medicine(s) being tested or used as alternative treatment(s);
• Splenectomy;
• First trimester of pregnancy.