natural immunity is when you get it then recover.. the fact a virus is "novel" or not is irrelevant.
With many viruses, immunity fades over time. Covid-19 seems to be an extreme example of this. But because it is a new, or novel virus, we really do not know how quickly immunity fades.
There is something going on here. For some reason, it is hitting countries in very quick waves, and even when it is assumed herd immunity is close, we just get a next wave. Assumably if herd immunity was around the corner, places that were ahead of us in the pandemic would be at herd immunity... But maybe we are the worst possible country, and just did not realize it.