Naturally acquired immunity to Plasmodium falciparum's asexual blood stage reduces parasite multiplication at microscopically detectable densities. The effect of natural immunity on initial prepatent parasite multiplication during the period following a new infection has been uncertain, contributing to doubt regarding the utility of experimental challenge models for blood-stage vaccine trials. Here we present data revealing that parasite multiplication rates during the initial prepatent period in semi-immune Gambian adults are substantially lower than in malaria-naive participants. This supports the view that a blood-stage vaccine capable of emulating the disease-reducing effect of natural immunity could achieve a detectable effect during the prepatent period.