What fraction of breakthrough COVID cases are attributable to low antibody count?

From the article:

Original antigenic sin, also known as antigenic imprinting or the Hoskins effect, refers to the propensity of the body's immune system to preferentially utilize immunological memory based on a previous infection when a second slightly different version of that foreign pathogen (e.g. a virus or bacterium) is encountered. This leaves the immune system "trapped" by the first response it has made to each antigen, and unable to mount potentially more effective responses during subsequent infections.

This was the first time I've encountered this concept. It actually made it seem like a booster shot would just be ineffective, rather than "seriously net-negative." Immunological memory would be optimized for the early variant the vaccine was designed for, and would be unable (or less able?) to update for the new variant.

Pfizer and Moderna vaccines target the spike protein. However, there are several other potential protein targets. Maybe the booster could be designed to target these instead.

That said, my read on the original antigenic sin article makes it seem like a plausible cause of breakthrough cases of Covid. If so, then I'd predict that antibody levels wouldn't be a good predictor of susceptibility to infection. But it would be much better to base this on empirical data, and I don't know if that exists.

There are probably some private meetings that would benefit from the antibody testing strategy. As you point out, personalized knowledge of the strength of your immunity would enable more precise risk analysis when planning get-togethers and outings in circumstances involving a lot of strangers.

Meetings in which all participants are known, and there is some level of trust or ability to enforce rules, might benefit more from rapid testing for Covid, rather than testing for antibodies. Of course, you could do both for the strongest level of confidence. But we have rapid tests available right now. It seems plausible that some combination of rapid tests, or PCR tests followed by rapid tests for large or high-stakes meetings, could provide enough security that further investments in safety would be best focused on circumstances where you have less control.

So it seems like antibody tests potentially permit more low-trust interactions, while infection tests permit more high-trust interactions. If antibody tests are known to be strong predictors of immunity, we can take them and decide whether to go to a music festival. If we're all trustworthy friends and all get tested daily for Covid, we can create our own mass Covid bubble and hold a big wedding.

I'm quite pessimistic about the likelihood that a meaningful amount of vaccine hesitancy is due to the perception that the vaccine's efficacy is too low to be worth bothering. There's just a collection of justifications floating around the antivax population, and they utter whichever one they've seen work to shut people up in the past.

But insofar as this perception is playing a role, identifying people whose vaccines "didn't take" and giving them targeted boosters might help. You could tell a story that we're trying to give people as little vaccine as possible - just to the people who we are confident really need it. Antibody tests might be a helpful supplement here. Some vaccine hesitant people might suspect they've already had Covid, in which case they might think there's less of a point in getting vaccinated (and as far as I know, they might be right?). If we can distinguish those who have no or low antibodies from those who have a higher level, perhaps this would sometimes create traction to motivate those with little-no resistance to get the jab. Then again, maybe they'd use greater evidence for fading immunity as a reason not to get vaccinated at all.

From this paper, young children have durably high antibody levels, while older children and adults have low or transient antibody levels. It suggests that the antibody transience in adults may be due to original antigenic sin, since we've had a lot more colds than children and may have already fine-tuned our immune systems to deal with those viruses, making them less able to adapt to handle Covid. This paper isn't the empirical study you'd really want, but it seems to support the idea that antibody level does control immunity to Covid, that antibody tests would be a useful way to check your immunity, and that adults really do need to worry about their immunity fading within months of getting vaccinated.

One of my remaining concerns with antibody tests is that they would need to be chosen to test against all strains of the virus. You don't want an antibody test that works against an early strain, but not against the most prevalent one. Right now, I have a BINAXnow rapid test in my closet, and it may well sit there for months unused until it expires. It wouldn't surprise me at all if a lot of people did the same thing with some sort of home antibody test kit, but instead of throwing it out at expiration (or after a shift in viral population), they used it anyway, and gained a false sense of security. So I'd probably want antibody tests to be administered by a medical professional, rather than in a home test.