LAUSR

Abstract The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to an unprecedented effort to generate real-world evidence on the safety and effectiveness of various treatments. A growing number of observational studies evaluating the effects of certain drugs have been conducted, including several assessing whether hydroxychloroquine improves outcomes in infected individuals and whether renin-angiotensin-aldosterone system inhibitors have detrimental effects. We review and illustrate how immortal time bias and selection bias were present in several of these studies. Understanding these biases and how they can be avoided may prove important for future observational studies assessing the effectiveness and safety of potentially promising drugs during the COVID-19 pandemic.