[hycaria]

1) It's not perfect immunity but rather resistance, and it's when they are heterozygous.

2) Heterozygous still carry the gene and can pass it on (you're likely to just exclude homozygous people)

3) Sickle-cell disease has a lot of downsides... Yes, it's a disease.

There is little to zero downside in preventing births of homozygous affected people.