Vacuum Polarization Effects in the Worldline Variational Approach to   Quantum Field Theory
  The worldline variational approach is extended beyond the quenched approximation, i.e. to include virtual pair production of heavy particles. This is achieved either by an expansion of the functional determinant to second order or by an hybrid ansatz for the quadratic trial action consisting of fields for the light particles and worldlines for the heavy ones as in the linear polaron model. Numerical results and analytic approximations show a reduction of radiative effects with increasing number of flavors. 
R. Rosenfelder