Adapted Live SARS‐CoV‐2 Vaccine Elicits Rapid Mucosal Immunity, Protects From Disease, and Reduces Shedding of XBB.1.5

ABSTRACT The emergence of SARS‐CoV‐2 variants, like XBB.1.5, causing immune evasion and frequent breakthrough infections, emphasizes the need for vaccines that limit transmission and target newly emerging variants. Mucosal vaccines, particularly live attenuated vaccines (LAV), are promising candidates for inducing strong mucosal immune responses to prevent viral replication and transmission. Vaccination with the previously described “one‐to‐stop” codon‐modified LAV OTS‐228, carrying the ancestral spike protein, induced sterilizing immunity against ancestral SARS‐CoV‐2 but also broad protection against Omicron variants, including XBB.1.5, but transmission of XBB.1.5 to contacts could not be prevented completely. As a proof‐of‐concept, we updated OTS‐228 by replacing the sequence coding for the ancestral SARS‐CoV‐2 spike protein with that of the XBB.1.5 variant. We applied flow cytometry to detect SARS‐CoV‐2‐specific T cell responses, as well as ELISA and qPCR, to characterize systemic and mucosal immune responses in Syrian hamsters in detail. The new OTS construct designated as “OTS‐300” exhibited an optimal safety profile in Syrian hamsters comparable to the original candidate vaccine. A single‐dose intranasal (i.n.) vaccination with OTS‐300 protects against disease, substantially limits XBB.1.5 replication, and reduces transmission in Syrian hamsters, showcasing the adaptability of the OTS platform for other emerging variants. OTS‐300 induced accelerated mucosal and systemic antibody responses and reduced virus‐mediated inflammation as compared with an intramuscularly delivered mRNA vaccine encoding the XBB.1.5 Spike.