Two models are presented for the description of the electron screening effects that appear in laboratory nuclear reactions at astrophysical energies. The two-electron screening energy of the first model agrees very well with the recent LUNA experimental result for the break-up reaction 3 He 3 He, 2p4 He, which so far defies all available theoretical models. Moreover, multi-electron effects that enhance laboratory reactions of the CNO cycle and other advanced nuclear burning stages, are also studied by means of the ThomasFermi model, deriving analytical formulae that establish a lower and upper limit for the associated screening energy. The results of the second model, which show a very satisfactory compatibility with the adiabatic approximation ones, are expected to be particularly useful in future experiments for a more accurate determination of the CNO astrophysical factors. PACS number(s): 25.10.+s, 25.55.-e, 25.45.-z,26.65.+t