Bivalent COVID-19 booster vaccines induce cross-reactive but not BA.5-specific antibodies in polyclonal serum

The question if the bivalent mRNA COVID-19 booster vaccination, containing wild type and BA.5 spike, provides enhanced benefits against BA.5 and similar Omicron subvariants has been widely debated. One concern was an ‘original antigenic sin’-like effect which may redirect immune responses to the bivalent vaccine towards the wild type spike and may block de novo generation of BA.5 specific antibodies. Here, we characterized the response to the bivalent vaccine and we performed antibody depletion experiments. Interestingly, when we depleted serum of all antibodies to wild type RBD, we also removed all reactivity to BA.5 RBD. This suggests that all antibodies induced by the bivalent vaccine – at least with the limit of detection of our assay in polyclonal serum - are in fact cross-reactive. This further suggests that, on a serum antibody level, the bivalent vaccine did not induce a de novo response to BA.5.