Background: COVID-19 has a broad clinical specturm. We investigated the role of serum markers measured on admission on severity as assessed at discharge and investigated those which relate to the effect of BMI on severity. Methods: Clinical and laboratory data form 610 COVID-19 cases hospitalised in the province of Zheijang, China was investigated as risk factors for severe COVID-19 (assessed by respiratory distress) compared to mild or common forms using logistic regression methods. Biochemical markers were correlated with severity using spearman correlations and a ROC analysis was used to determine the individual contribution of each of the biochemical markers on severity. We carried out formal mediation analyses to investigate the extent of the effect of body mass index (BMI) on COVID-19 severity mediated by hypertension, glycemia, Lactose Dehydrogenase (LDH) at the time of hospitalisation and C-Reactive Protein levels (CRP), in units of standard deviations. Results: The individual markers measured on admission contributing most strongly to prediction of COVID-19 severity as assessed at discharge were LDH, CRP and glucose. The proportion of the effect of BMI on severity of COVID-19 mediated by CRP, glycemia or hypertension we find that glucose mediated 79% (p