In summary, for a double-well system outside the cavity, the reaction takes place through the tunneling at the lowest two vibrational levels and a number of cotunneling processes. Each cotunneling process involves a pair of solvent-induced vibrational excitation and relaxation, bridged by the tunneling at the vibrationally excited state. The reaction rate is not solely determined by summing up these reaction pathways but also by their interference. For Model I with a much smaller molecule-solvent interaction strength λ i , or in some other system as demonstrated in Appendix A, it is interesting to note that including all cotunneling processes beyond the lowest pair of the vibrational levels actually decreases the reaction rate.