Numerous observational studies have demonstrated a relationship between the various white blood cell counts, most frequently neutrophils, lymphocytes, and eosinophils, and the severity of COVID-19. Since the research' goal was to forecast the prognosis, a causal connection was not required. However, causality becomes crucial if we start considering these biomarkers as possible therapeutic targets. Randomized trials are not always practical, and observational studies cannot demonstrate a causal link. Mendelian randomization studies, which are thought to be more reliable than observational studies in this situation, could strengthen the claims of causation. The issue of causality is not resolved in this case because two Mendelian randomization studies that looked for a link between the variety of white cell populations and COVID-19 severity produced inconsistent findings
