We have previously seen that those who were vaccinated after infection have much higher antibody levels than those who only got the vaccine or infection.
This study shows that it works the other way around, as well. People who are infected after vaccination (in this case elderly residents of a nursing home) had a dramatic increase in their antibody levels and neutralization titers.
Interestingly, none of them made antibodies to the nucleoprotein when they were infected. So that test can be used to identify people who were infected before vaccination but not those infected after vaccination. This isn't a bad thing; antibodies to the nucleocapsid don't really help you, because the protein is on the inside of the virus and so those antibodies can't prevent the virus from entering your cells. But it does make identifying people with infection after vaccination more difficult, since you'd have to have a positive antigen or PCR to know if you had a breakthrough infection.
The antibody boost is a silver lining to breakthrough infections. The outbreak in this nursing home was of the alpha variant. Most of those who were vaccinated but not infected had weak immunity against the alpha variant. But those who caught this variant were able to bring their neutralization titers for alpha up to levels similar to levels to the older variant. The immune system was able to "learn" alpha as a variant of something it had seen before and adjust to be able to better face it in the future.
We may be in for years of passing around the viruses, in spite of vaccination, but the ability of the immune system to adapt to new variants is encouraging.
Link to study: https://www.medrxiv.org/content/10.1101/2021.07.19.21260563v1.full.pdf