Sickle cell trait patient do not have manifestations as that of Sickle cell disease, because-
Sickle cell disease occurs in following forms


Heterozygous state, i.e., sickle cell trait.
Homozygous state, i.e., sickle cell anemia.


The most important factor for sickling is the amount of HbS in the cell.
In sickle cell trait (heterozygous state), only about 40% of the hemoglobin is HbS, this is insufficient to produce sickling manifestations.
In homozygous state, almost all the hemoglobin in the red cell is HbS, therefore they undergo sickling.
50% HbS is required for occurrence of sickling
HbA prevents sickling
50% sickles
HbA prevents polymerization of Hbs