Vaccination against COVID-19 increased immunity in the population, which reduced viral transmission and protected against severe disease. However, continuous emergence of SARS-CoV-2 variants required the implementation of bivalent boosters including the wild-type (D614G) and omicron (BA. 5) spike. Improved effectiveness of the bivalent booster versus monovalent booster against omicron subvariants has been reported; 1 however, few differences in the immune response have been detected. 2, 3

We investigated whether a bivalent COVID-19 booster vaccine that included wild-type spike and BA. 5 spike induced detectable BA. 5-specific antibody responses in serum. 16 serum samples collected at mean 31 days (SD 63 [range 0–260]) before and a mean 16 days (8 [6–31]) after receiving the bivalent booster were tested for antibody binding and avidity to the receptor binding domain (RBD) of wild-type and BA. 5 SARS-CoV-2. Neutralisation of wild-type and BA. 5 viruses was determined. Omicronspecific antibodies were measured by depletion of wild-type RBD reactive antibodies and assessment of depleted serum samples against BA. 5 RBD. A substantial increase in antibody binding to wild-type and BA. 5 RBD as well as in neutralisation of wild-type and BA. 5 viruses was seen in serum samples after receiving the bivalent booster (figure, appendix p 6). There were substantial differences in binding of post-booster serum samples between wild-type and BA. 5 RBDs; however, differences in neutralisation were not significant. Pre-booster and post-booster RBD antibody avidity