Here's my ELI5 attempt. Disclaimer: the immune system is very complex, even experts really don't understand it as much as we think we do. This is a very simplified view of the system, but I think captures a very large fraction of the story.

Let's focus on three types of antibodies - the things your immune system creates in response to a foreign object in your body - for this story:

1) IgM - this is created in your mucosal membranes when you breathe in something foreign. In the context of COVID, we'd expect to find IgM antibodies in people exposed to the virus because the mucosal membranes of the respiratory systems are the first places exposed to a respiratory virus.

2) IgG3 - this is created in your blood and is the main killer of a foreign object. This is what you think of when your immune system is fighting a virus. It also drives the inflammation which is what you generally recognize as the "flu-like symptoms".

3) IgG4 - this is created in response to what we recognize as allergens. For example, if you breathe in a bunch of pollen in the spring, you may have an IgG3 response and feel terrible. Or you body may recognize that that foreign object really isn't a big deal. In this case it will create IgG4 which basically tells your body to ignore the object until it's cleared. It's kind of the opposite of IgG3: rather than ramping up inflammation to fight the object, it's ramps it down and just says "nothing to see here" so you don't waste resources fighting something harmless.

What they found is that vaxxed and boosted people have a ton of IgG4 floating around and little to no IgM and IgG3.

So what does that mean? Here's where people are going to get spun up because we have to create hypotheses and test them. But even stating some hypotheses will cause the political machine to get mad.

1) IgM is really good at stopping viruses early. From this we'd expect that unvaxxed people exposed to COVID to be more resistant to future infections because the first exposure will create IgM and the virus will be stopped/attenuated in the nose. I don't know the state of the research on this, but it's a hypothesis to test.

2) IgG4 is a bad thing in the context of a replicating virus. If you have IgG4, it's telling your body to ignore the virus because it's "just an allergen". That's fine for pollen which will be cleared out of your system, but it's bad for a replicating virus because it can replicate unfettered. The hypothesis to test here is if people who are more vaxxed have more (re)infections of COVID. Another thought is that the infection may never go away and flare up occasionally (long COVID?). Again, I don't know the research, we need to test these hypotheses to understand it.

But it is one piece of the puzzle and a plausible explanation for this (https://www.medrxiv.org/content/10.1101/2022.12.17.22283625v...) where we see a nearly linear increase in vaccine doses and COVID infection. Again, these are just hypotheses and we need to test them without getting political about the vaccine.

This may also be a plausible explanation for COVID related OAS (Original Antigenic Sin) and ADE (Antibody Dependent Enhancement) that some researchers claim they see.

At a minimum we can say that the more vaxxed and boosted a person is, the more "non-standard" (compared to other vaccines) their immune response is, and that's worth further investigation.