Confirmed Plasmodium vivax resistance to chloroquine in Central Vietnam

Plasmodium vivax resistance to chloroquine (PvCQR) is currently reported in almost all vivax endemic countries. In Vietnam, despite a first report on PvCQR published in the early 2000s, P.vivax was still considered sensitive to CQ. Between May 2009 and December 2011, a 2-year cohort study was conducted in Central Vietnam to assess the recommended radical cure regimen based on a 10-day course Primaquine (0.5mg/kg/day) together with 3 days CQ (25mg/kg). We hereby report the results of the first 28-day follow-up estimating the cumulative risk of P. vivax recurrences together with the corresponding CQ blood concentrations among other endpoints. Out of 260 recruited P.vivax patients, 240 completed treatment and were followed up to day 28 according to the WHO guidelines. Eight patients (3.45%) had a P.vivax recurrent infection, at day 14 (n=2), day 21 (n=1) and day 28 (n=5). Chloroquine blood concentrations, available in 3/8 recurrent infections (day 14,21,28) were above the minimal inhibitory concentration (>100ng/ml whole blood) in all of them. Fever and parasitaemia (both sexual and asexual stages) were cleared by day 3. Anemia was common at day 0 (35.8%) especially in children below 10 (50%) and hemoglobin (Hb) recovery at day 28 was substantial among anemic patients (median change d28-d0 =+1.7g/dl; IQR[+0.7; +3.2]). This report, based on CQ blood levels measured at the time of recurrences, confirms for the first time P. vivax CQ resistance in Central Vietnam, and calls for further studies using standardized protocols for accurately monitoring the extent and evolution of PvCQR in Vietnam. These results, together with the mounting evidence of artemisinin resistance in Central Vietnam, further highlight the increasing threat of antimalarial drug resistance on malaria elimination in Vietnam.