Malaria attacks in children exposed to high transmission: who is protected?

In persons naturally exposed to high transmission, how significant, in terms of immune protection, is the occurrence or non-occurrence of a malaria attack during a given observation period? This question was studied in a West African village where Plasmodium falciparum malaria is holoendemic with intense and perennial transmission. A cohort of 94 children aged 4 months-14 years from Dielmo village, Sénégal, was studied over 4 months, June-September 1990. 41 children had no malaria attack and 53 children suffered between one and 6 attacks. The average number of attacks was 1.80, 2.25, 1.87, 0.29 and 0.07, respectively, in children aged 4-11 months, 1-2, 3-6, 7-10 and 11-14 years. The transmission level was 75 infective bites per person. Analysis of the distribution of the number of attacks in individual children suggested that all children within the same age group had either the same, or a very similar, level of protection. This suggests that the acquisition of clinical protection in areas where malaria is highly endemic involves a progressive and homogeneous decrease of the probability of having a malaria attack (attacks occur less frequently as age increases, in all children), rather than the acquisition of complete protection by an increasing number of children. The differences between the number of clinical attacks observed in young children and their presumed exposure suggest that protective mechanisms become effective from the first reinfection onwards, and are independent of the cumulative exposure to a great variety of antigens.