SARS-CoV-2 continues to evolve into distinct sublineages by acquiring additional mutations (see section 3.2). Although real-world effectiveness studies suggest that the current bivalent vaccines continue to provide protection against other circulating sublineages of Omicron, including the XBB–XBB.1.5 (Lin et al. 2023; Link-Gelles et al. 2023), there appears to be an inverse relationship between the time since vaccination and vaccine effectiveness, such that bivalent COVID-19 vaccine effectiveness against Omicron sublineages appears to wane over time (Link-Gelles 2023). Additionally, studies indicate that neutralizing antibody titers induced by the current bivalent COVID-19 vaccines against XBB-XBB1.5-related sublineages are lower relative to neutralizing antibody titers induced against the matched BA.4/BA.5 sublineage (Jiang et al. 2023). These data suggest that an updated strain composition of COVID-19 vaccines to more closely match currently circulating Omicron sublineages is warranted for the 2023–2024 vaccination campaign”
www.fda.gov/media/169378/download
h/t A Deplorable Neanderthal