Reinfected! and the reinfection becoming a bad case! No immunity... Article hints that you can expect to be relatively protected for up to 6mo after infection.. But there are no guarantees.
From the article: "There is still a lot for us learn before we have a full understanding of how immunity to COVID-19 works. (..) we all should still follow Government guidelines on social distancing to ensure we play our part in minimising the spread of COVID-19 within our communities." -Professor Paul Moss