We developed an analytic theory for the VSC-modified rate constant kVSC (Eq. 17) for a single molecule strongly coupled to the cavity, under the lossless regime (when τ -1 c ≪ Ω R ). This analytic theory is based on the mechanistic observation of sequential rate-determining steps |ν L , 0⟩ → |ν L , 1⟩ → |ν ′ L , 0⟩ (outlined in Eq. 11), which are observed in our numerically exact quantum dynamics simulations (see Supplementary Note 4). The theory kVSC (Eq. 17) explains the resonance condition ω c = ω 0 and the close connection between the rate constant modification kVSC and the optical lineshape A ν (ω -ω 0 ) (Eq. 14). This explains why the VSC-modified rate distribution closely follows the optical spectra as observed in the VSC experiments . This analytic theory kVSC provides accurate ω c -dependence of the VSC rate constant enhancement compared to the numerically exact results from HEOM simulations.