HbAS children had lower parasite densities than HbAA children at younger ages. The y-axis on the left denotes geometric mean parasite density as measured by blood smear. The y-axis on the right denotes relative parasite density of HbAS versus HbAA children. The light blue band indicates the 95% CI for the relative density; protection is significant at the P = .05 level for ages where the band does not cross the dotted blue line. Data were analyzed using generalized estimating equations with robust inference.