Intranasal DC-targeting vaccine booster elicits durable and cross-clade protective immunity against sarbecoviruses in mice.

Short-lived, clade-specific immune responses with limited mucosal priming are limitations of current COVID-19 mRNA vaccines. We have developed a nasal booster vaccine candidate that induced robust, sustained, cross-clade, systemic, and mucosal protective immunity. Two recombinant Clec9A-specific monoclonal antibodies fused to the receptor binding domain (RBD) from Omicron XBB.1.5 and SARS-CoV-1 were generated. In Comirnaty mRNA-vaccinated mice, boosting with both constructs combined (Clec9AOMNI) induced cross-clade neutralizing antibodies and T cell responses that were greater in magnitude and more sustained compared with bivalent Comirnaty (BC) mRNA vaccine booster. Persistence of RBD-specific follicular helper CD4+ T cells, germinal center B cells, and long-lived plasma cells that facilitated affinity maturation correlated with detection of triple cross-reactive B cells binding the RBDs of ancestral SARS-CoV-2, XBB.1.5, and SARS-CoV-1. Remarkably, intranasal boosting with Clec9AOMNI elicited robust and durable immunity across the upper and lower airways while concurrently boosting the systemic immunity to levels matching or exceeding those from systemic boosting. Correspondingly, Clec9AOMNI nasal booster conferred superior protection against SARS-CoV-2 challenge compared with BC mRNA booster, with undetectable viral titers in the respiratory tract. Hence, Clec9AOMNI is a promising nasal booster vaccine candidate that has the potential to mitigate pandemic threats from emerging sarbecoviruses.