The jury is still out on how protective IgG is for COVID. I know a few labs have LDT's and even the big players like Quest, ARUP, etc are only just coming up with serology. Most vendors I have seen just have a qualitative assay. I'm sure we will get a quantitative assay down the road.
Is it protective? No one knows
Is there a certain titer that gives immunity? No one knows
This is all way too early to tell. I have seen some neutralization studies that are promising and there was a study in macaques that showed high levels of IgG with a protective response a month later after injection with the virus. However, will this translate to humans no one knows.
I'm not an expert on this, but I assume we can answer this question with studies that follow patients with high titers.
Many patients with SARS-1 had high IgG levels for years after exposure (but again does that mean immunity?).
Will this act like HIV or HCV where it is a marker of exposure? Or will it act like HBV or measles?
The CDC, NIH, academic labs, etc will hopefully answer these questions in the next few months.