It is as yet unclear how long effective immunity lasts after catching the disease, but pessimistic estimates put it at less than six months. While the pessimistic estimates are likely wrong (hence the term "pessimistic"), there certainly isn't confidence in the sort of lifelong immunity that one gets to some other contagions.

Part of what's so exciting about the vaccines is that the antibodies seem to decay at a slower rate, meaning that it could result in longer immunity (though we obviously don't know for sure yet).