Humoral Responses Elicited after a Fifth Dose of SARS-CoV-2 mRNA Bivalent Vaccine.

While an important part of the world's population is vaccinated against SARS-CoV-2, new variants continue to emerge. We observe that even after a fifth dose of the mRNA bivalent vaccine, most vaccinated individuals have antibodies that poorly neutralize several Omicron subvariants, including BQ.1.1, XBB, XBB.1.5, FD.1.1, and CH.1.1. However, Fc-effector functions remain strong and stable over time against new variants, which may partially explain why vaccines continue to be effective. We also observe that donors who have been recently infected have stronger antibody functional activities, including neutralization and Fc-effector functions, supporting the observations that hybrid immunity leads to better humoral responses.