Lennie is an ICU consultant at UMC Utrecht, with a background in infectious diseases and epidemiology. She co-leads (with Olaf Cremer) the "epidemiology of sepsis and inflammation" research group. Her research focuses on clinical trials involving critically ill patients, particularly those with sepsis and severe infections, to enhance the methodology, design, delivery, and analysis of trials. Lennie serves as the European PI and immediate past global chair of REMAP-CAP, and is a co-applicant and work package leader for EU-funded projects such as RECOVER and ECRAID-Base. She also leads the ZonMw-funded IMPRINT consortium and is a co-applicant on several international collaborative grants. Additionally, she is a member of the Steering Committee for the Surviving Sepsis Campaign and the ISAB of Sepsis Canada. Since 2026, she is the CEO of Ecraid, a not-for-profit research organization leading and delivering research on infectious diseases in Europe.

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