I don't know of any study of the Delta variant (or other recent variants) comparing immunity acquired from infection versus immunity conferred by vaccination -- AFAIK, it's still unknown how these compare and whether a vaccination might provide additional immunity against variants. IIRC, the mRNA vaccines were developed to target key parts of the virus that are thought to be unlikely to mutate without decreasing virus viability.
We agree on this.
> I don't know of any study of the Delta variant (or other recent variants) comparing immunity acquired from infection versus immunity conferred by vaccination -- AFAIK, it's still unknown how these compare and whether a vaccination might provide additional immunity against variants.
I'm not aware of any study of this, either. So far, everyone is focusing on the vaccines.
On the contrary, from all that we know today, wouldn't we expect people with high IgA titres to even develop neutralizing immunity, at least in the weeks after titres peak?
Antibody titres aren't constant. They increase when your immune system sees an antigen, and decline gradually afterward. In individuals with previous exposure to the antigen, they increase more quickly due to immune memory. This is true for any virus. Decline in circulating antibodies does not mean that your immunity has gone away.
Vaccination is, to a first approximation, injecting a large amount of antigen into someone to induce an immune response. Therefore, data that shows that previously exposed people have faster and stronger immune responses to vaccination are simply confirming what we already know about immunity. It does not mean that the vaccination made the response in exposed people stronger than it would have been otherwise.
Maybe it did, but you can't conclude it from this kind of data.
Most notably, even those previously infected people with ~0 titers at baseline rapidly developed high levels of antibodies at 5-8 days after vaccination. It's a small group, but the rapid nature of the response is telling.
This data is clear that the infected people had a rapid, robust immune response to vaccination. It cannot tell you what you're trying to infer -- that the vaccination itself increased the immune response. To determine that, you'd need to do some other experiment.