Intranasal booster with SARS-CoV-2 RBD protein fused to E. coli enterotoxin a subunit after primary mRNA vaccination in mice.

The outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 2019 led to the coronavirus infection diseases 2019 (COVID-19) pandemic, significantly impacting global public health and the economy. Numerous COVID-19 vaccines based on the receptor binding domain (RBD) of SARS-CoV-2 spike protein have been developed, utilizing various protein expression platforms and adjuvant systems. In a previous study, we reported using the direct fusion of the A subunit of type IIb E. coli heat-labile enterotoxin with the SARS-CoV-2 RBD protein (RBD-LTA) as an intranasal vaccine candidate (Hsieh et al., 2023). In this study, we investigated the effects of an intranasal booster of RBD-LTA/RBD mixture proteins after one or two doses of intramuscular bivalent BA.4/5 mRNA vaccination over 17 and 35 weeks. Our results indicate that the intranasal RBD-LTA/RBD mixture proteins booster maintains high levels of anti-RBD IgG and neutralizing antibodies, comparable to those elicited by a two-dose mRNA vaccination regimen. An additional RBD-LTA/RBD mixture proteins booster significantly increased antibody titers, demonstrating the potential of this approach for long-term immunity against SARS-CoV-2. Our findings suggest that combining primary mRNA vaccination with an intranasal RBD-LTA/RBD mixture proteins booster can effectively sustain antibody levels over extended periods, providing a promising strategy for long-term protection against SARS-CoV-2 and its variants.