In 2008 Collen reached the mandatory retirement age of 65 years as Professor of the Faculty of Medicine at KU Leuven (Belgium), where he served as Director of the Center for Thrombosis and Vascular Research (now Center for Molecular and Vascular Biology) and the VIB Department for Transgene Technology and Gene Therapy (now VIB-KU Leuven Center for Cancer Biology). He authored and co-authored over 650 research papers in peer reviewed international journals which have been cited over 70,000 times, and 39 US Patents. He ranked among the 100 most cited scientific authors of the 1980s listed by Current Contents and among the top 100 living contributors to biotechnology in polls conducted by Reed Elsevier in 2005. In 2012 SciTech Strategies placed him among the top 400 most influential scientists in biomedical research for the period 1996–2011.