Temperature-dependent wave function renormalization and weak interaction rates prior to nucleosynthesis

We present a general prescription for the effects of medium-dependent wave function renormalization on rates of reactions involving a Dirac particle. The result is applied to the corrections of order e(2) to the weak interaction rates prior to nucleosynthesis, due to the thermal photon and e(+)e(-) baths. The results coming from the standard ''finite temperature'' corrections in the literature, a decrease of the predicted He abundance by 0.16%, are found to be replaced by an increase of 0.02%. But there are indications that plasma corrections that are not included at the e(2) level may be more important than these corrections.