It doesn't appear to be a reinfection. it looks like there's a couple of things that might be happening. Firstly, the oral swabs have limitations. Previous research has shown that as the illness progresses the cells in a patients intestines become affected. It then becomes possible to get a negative result from an oral swab and a positive result from an anal swab. The virus is still there but it's no longer detectable in the respiratory tract. This actually creates another avenue for transmission via the fecal-oral route. Something that has previously been the case for other Zoonotic viruses. In actual fact it's also possible to have negative tests for both swabs but the virus might still in the bloodstream. The second thing that appears to be happening is that since the virus was never completely gone it flairs up again when the patient is no longer receiving aggressive hospital treatment. There's nothing to indicate that a vaccine wouldn't work, but of course the vaccine needs to matched to the current strain of the virus. As viruses mature the strain tends to become more stable.