His current work focuses on infectious disease and antimicrobial resistance modelling, randomised trial designs, economic evaluations alongside trials, incorporating long-term effects into economic evaluations, and statistical approaches to address time-dependent confounding in burden of illness studies.

Koen is a member of the Healthcare Associated Infectious Working Group of the Joint Committee on Vaccination and Immunisation (JCVI). His work has directly informed national vaccination and immunisation policies, as well as national targets for antibiotic prescribing in primary care.