I briefly skimmed the article at the medrxiv link, but it does not seem to assay neutralizing antibody titer. Rather the medrxiv link just assays "reactivity."

I briefly skimmed the article at the medrxiv link, but it does not seem to assay neutralizing antibody titer. Rather the medrxiv link just assays "reactivity." I suppose it would be great if the booster increased reactivity against all variants. But it is not so surprising that a booster furthers the selection of higher affinity antibodies, which tend to be more specifically adapted to the antigen. However, at the end of the day, the Pfizer data shows that either this decrease in reactivity is not enough to overcome the recovery in titer elicited by the booster or that the decrease in reactivity is not a general phenomenon. Or, hopefully not the case, we will start to see this decrease in reactivity effect as we look at larger sample sizes. I think there is a ton of data available to Pfizer about booster titers/effectiveness, so I would be surprised to see larger sample sizes change the conclusions.