Impact of Chemoprophylaxis on Plasmodium vivax and Plasmodium ovale Infection Among Civilian Travelers: A Nested Case-Control Study With a Counterfactual Approach on 862 Patients.

This article examines the impact of chemoprophylaxis on the clinical course of imported malaria caused by *Plasmodium vivax* and *Plasmodium ovale*—species known to form dormant liver stages called hypnozoites—in civilian travelers to France from 2006 to 2017. The study found that travelers who used chemoprophylaxis, particularly blood-stage–acting drugs such as doxycycline, mefloquine, chloroquine, or chloroquine-proguanil, had a significantly increased risk of delayed symptom onset beyond 60 days after return, whereas atovaquone-proguanil, which also targets liver schizonts, was not associated with such delay. Other postinfection outcomes, including severity, hospitalization rates, and biological markers, were not significantly affected by prophylaxis use. These findings highlight the limitations of current blood-stage–only chemoprophylaxis in preventing relapses caused by hypnozoites and suggest the need for new prophylactic strategies that target both erythrocytic and liver stages of malaria.