According to top health officials in Norway (as seen on a TV program last night), reinfection is not likely, at least in the short run. And the Japanese case apparently is based on a misunderstanding, not an actual reinfection.

Over the long haul, reinfection is quite possible, since immunity wears off with time. It is not considered unlikely that this disease will be with us forever. In that case, vaccination will perhaps be our best tool to manage it.