Its pretty simple, antibodies are egg timed they swine we know over a six month period. Then you are relying on T-Cells, unless you top up your antibodies through boosting.
The severity of having Covid, if you've had once isn’t predictable, it’s can be down to a range of factors but particularly viral load, plenty of young people in ICU and stats in mortality, more so in the unvaccinated.
There was something on ITV news a week back, from a hospital in Liverpool where the biggest cohort they had in ICH were unvaccinated young adults.
In a weird way, younger is now the new vulnerable, particularly children, but young adults too as they are the biggest cohort who may be unvaccinated, had a single dose, or waiting for boosters etc.