Conceptual causal framework to assess the effect of SARS-CoV-2 variants on COVID-19 disease severity among hospitalized patients, Van Goethem, Nina, Serrien Ben, Vandromme Mathil, Wyndham-Thomas Chloé, Catteau Lucy, Brondeel Ruben, Klamer Sofieke, Meurisse Marjan, Cuypers Lize, André Emmanuel, et al. , Archives of Public Health, Jan-12-2021, Volume 79, Issue 1, (2021)