When it comes to coronavirus and biology - sex matters Scott M Nelson, MD PhD1,2Martina Capuzzo3Antonio La Marca, MD PhD31School of Medicine, University of Glasgow, UK2NIHR Bristol Biomedical Research Centre, Bristol, UK3Department of Medical and Surgical Sciences for Children and Adults, University of Modena and Reggio Emilia, Modena, ItalyCorresponding author:Prof Scott M Nelson University of Glasgow New Lister Building Glasgow Royal Infirmary Glasgow G31 2ER, UK +44 (0)141 201 8581 [email protected] count: Text 1058The male dominance in COVID19 morbidity and mortality figures has been a consistent feature from the early reports emerging from Wuhan, to national intensive care data, through to more recent comprehensive population mortality data. Men are more than twice as likely to become severely ill and require intensive care than women and at least twice as likely to die, with further widening of the mortality discordance with increasing age. These differences do not appear to be caused by differential rates of infection, as equal numbers of men and women catch SARS-CoV-2. Nor do they appear to reflect clustering of unhealthy behaviour and comorbidities in men, as even after adjustment, male sex is associated with a hazard ratio (HR) of death from COVID19 of HR 1.99, (95%CI 1.88-2.10)1. These sex-specific differences in severity and fatality were also observed for the 2002-2003 SARS-CoV outbreak, and the Middle East respiratory syndrome (MERS)-CoV2. So, what underlies this sex-specific susceptibility difference for pathogenic coronaviruses? Several early lifestyle theories were proposed, but rather more fundamental sex-specific discordances in steroid hormones, x-linked genes and the innate immune response are likely to underlie this sexual dimorphism.