A 28-year-old female from coastal Tamil Nadu presents to a government hospital with fever, headache, and myalgia for 4 days. She lives in a fishing hamlet near a lagoon with stagnant water. On blood smear, *Plasmodium vivax* is identified. She is treated with chloroquine and primaquine. On day 3 of treatment, fever resolves and parasitaemia clears. However, she returns 6 weeks later with recurrent fever and parasitaemia. Which epidemiological and parasitological feature best explains this recurrence in the coastal endemic zone?