Figure 1.

Annual incidence rate of clinical Plasmodium falciparum episodes per 100 children (bold line). In order to overcome the fluctuations of the annual incidence rate, we fit second (dotted line), third (dashed line) and fourth (solid line) degree polynomial trend lines to the data (bold line). The corresponding R² values are 0.70, 0.91 and 0.99, respectively, indicating an accurate fit for third and fourth order polynomials. The inflexion on these two trend lines indicates the onset of acquisition of clinical immunity at approximately 3–4 years of age.