I'm not an immunologist or even a biologist, but: biological lifeforms are complex things that don't really work in absolutes, but in shades of gray. Having ant

I'm not an immunologist or even a biologist, but: biological lifeforms are complex things that don't really work in absolutes, but in shades of gray. Having antibodies and so on is not an absolute protection from reinfection, but rather helps fight it. If there aren't enough of them, or in various other circumstances (suppressed immune system, different virus variant, etc), it seems only natural that it is possible to be infected again.