CONCLUSIONS: In children with severe hemophilia A, once-weekly prophylaxis with efanesoctocog alfa provided high sustained factor VIII activity in the normal to near-normal range (>40 IU per deciliter) for 3 days and more than 10 IU per deciliter for almost 7 days after administration, leading to effective bleeding prevention. Efanesoctocog alfa was associated with mainly nonserious adverse events. (Funded by Sanofi and Sobi; XTEND-Kids ClinicalTrials.gov number, …