To summarize, we present a microscopic mechanism and analytic rate theory that explain resonance VSC suppression of the rate constant under the normal-incidence condition, collective light-matter coupling, and thermally activated regime. In this mechanism, the cavity mode q c is coupled to the solvent vibrations {Q j }, which in turn coupled to a reaction coordinate R 0 . This collective coupling can significantly influence the reaction rate constant of the reaction coordinate. For the model reaction considered in this work, the reactant vibrational excitation |ν L ⟩ → |ν ′ L ⟩ is the rate-limiting step and thus controls the overall apparent rate constant. In the current mechanism (Eq. 7), coupling to the cavity explicitly splits the effective solvent spectral density that influences the dynamics of reaction coordinate (see Fig. ), and thus reduces the thermal activation rate k 1 , hence reducing the overall apparent rate constant.