Apparently this question is more complicated than a quick Google search can handle and, while I did find a very promising article/medical study, it wasn't free but available only for $31.50.

One thing that stood out to me from another article I found, was: "Surgical stainless steel rarely causes allergic skin reactions; however, not all stainless steel products are nickel-free." So perhaps that is a factor?

Also, where is the piercing? In the flesh or your ear, the cartilage, what? The location can have an effect on the rate of recurrent infection.