At the University of Iowa, the investigators led a multicenter randomized clinical trial comparing standard prophylactic dose to intermediate dose enoxaparin in hospitalized patients with COVID-19 (NCT04360824). As part of an exploratory biomarker component of this trial, blood samples were collected from hospitalized COVID-19 patients at enrollment and weekly for up to 30 days of hospitalization. The pilot results, as well as reports from other groups, demonstrate increased potential for thrombin generation in the plasma of COVID-19 patients. In particular, in the COVID-19 patient cohort enhanced thrombin generation potential persisted for at least 30 days of hospitalization. The investigators now propose to explore the mechanistic roles of activation of blood cells (such as platelets and neutrophils) and products of cellular activation as mediators of enhanced thrombin generation in patients with COVID-19. The study design will be a longitudinal cohort study, which will allow for the determination of the time course of enhanced thrombin generation potential in relation to clinical outcomes and changes in markers of cellular activation in serial samples obtained from COVID-19 patients for up to 3 years after infection with SARS-CoV-2. This study may provide clues to why a subset of COVID-19 patients present with late thrombotic complications even after apparent recovery from SARS-CoV-2 infection. An ongoing question in the field relates to the comparative prothrombotic effects of acute COVID-19 versus incidental SARS-CoV-2 infection versus acute infection with influenza viruses. Therefore, we will include three categories of hospitalized patients in this study: (1) acute COVID-19, (2) incidental COVID-19, and (3) acute influenza A or B. This project has a strong scientific rationale with direct clinical implications, especially given the emergence of SARS-CoV-2 variants such as delta and omicron that may prolong the pandemic and/or cause surges of COVID-19 in the coming months.