Boosting hybrid immunity with intranasal live-attenuated or intramuscular mRNA vaccination prevents SARS-CoV-2 XBB.1.5 transmission

Rapid antigenic evolution of SARS-CoV-2 and waning immunity have reduced antibody-mediated neutralization, enabling breakthrough infections and sustained transmission despite widespread immunity. This highlights the need for variant-adapted booster vaccines. However, pre-existing immunity may influence the magnitude and specificity of immune responses induced by variant-adapted booster vaccination. Using the Syrian hamster model, we previously described a safe and efficient mucosal vaccination approach using live attenuated viruses (LAV) based on a "One-to-Stop (OTS)" codon modification strategy. Whether OTS vaccines are suitable as effective booster vaccines in the presence of pre-existing mucosal immunity remained unaddressed. Here, we modeled pre-existing hybrid immunity in Syrian hamsters using intramuscular bivalent mRNA vaccination followed by SARS-CoV-2 infection and subsequently administered heterologous intranasal OTS-LAV or intramuscular mRNA XBB.1.5 booster vaccination. Both booster strategies significantly increased XBB.1.5-binding- and neutralizing antibodies systemically and at mucosal sites. In contrast, booster vaccination left SARS-CoV-2 specific T cell frequencies unchanged. While all hybrid-immune animals were protected from disease following XBB.1.5 challenge infection, virus transmission was completely blocked only after booster vaccination and was associated with the absence of infectious virus in nasal samples. Collectively, using OTS-LAV we provide a proof of concept that intranasal LAV vaccination can effectively boost pre-existing hybrid immunity and restrict viral transmission. These findings support the use of OTS-LAV as a heterologous booster strategy against SARS-CoV-2 and demonstrate that variant-adapted booster vaccination can efficiently limit transmission, a strategy highly desired for tackling emerging respiratory infections.