[geofft]

Spanish flu (influenza A) was not a coronavirus. Our anonymous friend's assertion is that extrapolations from influenza A don't apply to SARS-CoV-2, which is a coronavirus (otherwise we'd expect e.g. an annual coronavirus flu season with new strains every so often, much the way there's an annual influenza A season). That much, at least, appears to be correct.

I don't disagree it's a possibility, but influenza A isn't particularly informative on how likely the possibility is.

[ncmncm]

The likelihood of a mutation that evades (or is more severe because of) immune response to previous infections is directly proportional to the number of cases, i.e. is growing exponentially.

There is some evidence that prior exposure to other coronavirus (including vaccination) is an additional risk factor for loss of lung function.

So, yes, the influenza A experience is relevant. In that instance, those with a strong immune response had a greater fatality rate.