A 28-year-old female from Mumbai, who had returned from a 2-week business trip to Chhattisgarh 6 weeks ago, presents with fever, headache, and generalised myalgia for 4 days. She had taken chloroquine prophylaxis during her stay but did not complete the post-travel course. Thick blood smear shows P. vivax parasites. She has no splenomegaly. What is the MOST likely epidemiological explanation for her infection despite prophylaxis?