PSMD1 and COVID-19: Notably, in this head-to-head comparison of COVID-19 booster vaccines on SARS-CoV-2 onward transmission, we found that statistically significant viral reduction in the lung tissues and oral swabs was observed only in the intranasal S1 nanoparticle vaccine-boosted group, but not in the systemic mRNA vaccine-boosted group, suggesting the superior protection of this intranasal vaccine, which could act as an attractive vaccine booster candidate to complement the current licensed systemic vaccines.