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Plasmodium vivax causes >80% of malaria cases in Brazil. Management of vivax malaria requires eliminating blood-stage parasites as well as hypnozoites, dormant liver parasites that cause relapse. The currently recommended treatment in Brazil (chloroquine plus primaquine) is only 60% to 70% effective in preventing relapse. In a trial in the Brazilian Amazon, investigators compared the recommended total primaquine dose with double that amount. Participants included 254 patients aged ≥5 years (median age, 22; 55% male) with microscopy-confirmed vivax malaria who had received directly observed chloroquine for 3 days (total dose, 25 mg/kg); all were blood-smear–negative by day 7. Patients were randomized to: (1) unobserved primaquine (total dose…