A 45-year-old woman presents to a London tropical disease unit 10 days after returning from a 3-week holiday in Goa, India. She took no malaria prophylaxis. She reports cyclical fevers every 48 hours with drenching sweats and chills. Examination reveals mild splenomegaly but she is otherwise well. Thick and thin blood films demonstrate P. vivax trophozoites with parasitaemia of 0.3%. G6PD activity is normal. After treatment of the acute episode with chloroquine, what additional treatment is required to prevent relapse?