We study mortality impacts of sudden stratospheric warmings (SSWs) via counterfactual ensemble simulations. SSWs robustly increase the likelihood of cold spells in Northern Europe, but the socio-economic effects remain poorly quantified. Using re-forecasts of the 2018 SSW and mortality records, we attribute hundreds of expected deaths in the UK and Nordics to this event. The strongest impacts emerge from combined post-SSW cold anomalies and heightened mortality sensitivity to these anomalies.