Therapeutic and prognostic role of vitamin D for COVID-19 infection: A systematic review and meta-analysis of 43 observational studies. - Petrelli, Fausto; Luciani, Andrea; Perego, Gianluca; Dognini, Giuseppina; Colombelli, Paolo Luigi; Ghidini, Antonio (Journal of Steroid Biochemistry & Molecular Biology, 2021)