SARS immunity lasted years. We don’t know how long COVID-19 immunity lasts, but we do know that in resus macaques it lasted some period of time.
The ugly truth is that if immunity to this doesn’t last:
A) There will never be a vaccine, and
B) There will never be containment.
Which basically means we just need to buckle up for everyone who could die from it to die from it, and the measures we’re taking to crater the world economy to stop it aren’t going to help anyway.