We have modified the standard code for primordial nucleosynthesis to include the effect of the slight heating of neutrinos by $e^pm$ annihilations. There is a small, systematic change in the $^4$He yield, $Delta Y simeq +1.5times 10^{-4}$, which is insensitive to the value of the baryon-to-photon ratio $eta$ for $10^{-10}la eta la 10^{-9}$. We also find that the baryon-to-photon ratio decreases by about 0.5% less than the canonical factor of 4/11 because some of the entropy in $e^pm$ pairs is transferred to neutrinos. These results are in accord with recent analytical estimates.
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