From what I've read there isn't a definitive answer for this.
If you read science articles then the jury is still out, words like 'may' and 'might' are used.
The NHS and CDC say you should still get the vaccine, but then they would say that, other scientists say different as is normal in any other medical situation.
With both natural immunity and vaccination the odds of future infection and level of illness are both dependent on age, comorbitities ect
Surely if vaccines were the only way to secure longer immunity, then proof of previous covid infection wouldn't be used on the domestic passports
If a business owner does try playing that card then they need to tread carefully as vaccines aren't mandated and legally they might be open to being sued