HbAS children had a lower molecular force of infection than HbAA children at older ages. The y-axis on the left denotes the number of incident P falciparum strains detected in the blood per year, or the molecular force of infection. The y-axis on the right denotes the IRR of the molecular force of infection in HbAS versus HbAA children. The light blue band indicates the 95% CI for the IRR; protection is significant at the P = .05 level for ages where the band does not cross the dotted blue line. Data were analyzed using negative binomial regression.