unlike sickle cell anaemia in humans, which is a classical resistance factor because it reduces malaria parasite densities
http://plosbiology.org/article/info:doi/10.1371/journal.pbio.1000004#article1.body1.sec3.p1

Contrary to popular belief, sickle cell appears to be a tolerance factor to infection more than a resistance factor from infection. That is, it tends to reduce the severity of symptoms more than reduce actual parasite numbers.

See:
Wellems TE, Hayton K, and Fairhurst RM. 2009. The impact of malaria parasitism: from corpuscles to communities. J Clin Invest 119(9):2496-2505.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2735907/