  We theoretically investigate the polariton-polariton interaction in
microcavities beyond the commonly used Born approximation (i.e., mean-field),
by adopting a toy model with a contact interaction to approximately describe
the attraction between electrons and holes in quantum well and by using a
Gaussian pair fluctuation theory beyond mean-field. We obtain a density or
chemical potential independent polariton-polariton interaction strength even in
two-dimensions, which result from coupling to the photon field. We show that
quantum fluctuations lead to about a factor of two reduction in the
polariton-polariton interaction strength within our toy model. Together with
corrections to the 1s exciton approximation at very strong light-matter
coupling, we find the polariton-polariton interaction strength under typical
experimental conditions is overestimated by a factor three in the widely used
theories, if our toy model can qualitatively simulate the polariton interaction
in GaAs quantum wells. We compare our prediction with the most recent
measurement and argue that the beyond-Born-approximation effect to the
polariton-polariton interaction strength is crucial for a quantitative
understanding of the experimental data by E. Estrecho \textit{et al.}, Phys.
Rev. B \textbf{100}, 035306 (2019).
