It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle cell anaemia is persisting in human population. Why ?

The harmful alleles get eliminated from population over a period of time, yet sickle cell anaemia is persisting in human population because $SCA$ is a harmful condition which is also a potential saviour from malaria. Those with the benign sickle trait possess a resistance to malarial infection. The pathogen that causes the disease spends part of its cycle in the red blood cells and triggers an abnormal drop in oxygen levels in the cell. In carriers, this drop is sufficient to trigger the full sicklecell reaction, which leads to infected cells being rapidly removed from circulation and strongly limiting the infection's progress.

These individuals have a great resistance to infection and have a greater chance of surviving outbreaks. This resistance to infection is the main reason the $SCA$ allele and $SCA$ disease still exist It is found in greatest frequency in populations where malaria was and often still is a serious problem