You are overestimating the "research shows" part, and no research shows that "natural immunity" is achieving a higher than 90% immunity against any infection.

Nor is it clear who actually has a "more effective" or even "sufficiently effective" immunity, given that the infection could be up to a year or more ago, and almost nobody knows exactly what strain they got. And no, you can't tell any of that easily, reliably or cheaply by antibody testing.

Proof of vaccination is the cheapest and most reliable proof of a reasonable chance of immunity.