TLDR: My read of the underlying study is that the second booster isn't as effective as the first, and they say it's probably due to the body being overexposed to the vaccine, creating the impression to the immune system that C19 is no longer a deadly virus, but a mere allergen, so it accepts the virus. i.e. the vax makes you get covid more easily / instead of being vaccinated from it, you are being trained to not fight against it.