The Tommerup group has more than 40 years of experience within medical and human genetics, ranging from the pre-genomic era of cytogenetics to the present genomic revolution mediated by NGS/bioinformatics. The present SARS-CoV-2 pandemic has revealed a striking excess of severe COVID-19 deaths among males, and a striking excess of females with persistent COVID symptoms. Moreover, influenza morbidity and mortality are often higher for women than men and a male predominance is seen in RSV morbidity among children. In REACT we will search for viral disease specific and shared host genetic variants that may be associated with these sex differences, with a specific focus on the X and Y chromosomes and on differentially expressed genes in males and females.