The interaction rate of an ultrarelativistic active neutrino at a temperature below the electroweak crossover plays a role in leptogenesis scenarios based on oscillations between active neutrinos and GeV-scale sterile neutrinos. By making use of a Euclideanization property of a thermal light-cone correlator, we determine the $O(g)$ correction to such an interaction rate in the high-temperature limit $pi T gg m_W$, finding a $sim 15 ... 40%$ reduction. For a benchmark point, this NLO correction decreases the lepton asymmetries produced by $sim 1%$.
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