A mathematical model for SARS-CoV-2 in variable-order fractional derivative Mahmoud H. DarAssi, Mohammad A. Safi, Muhammad Altaf Khan, Alireza Beigi, Ayman A. Aly and Mohammad Y. Alshahrani Eur. Phys. J. Spec. Top., 231 10 (2022) 1905-1914 Published online: 03 February 2022 DOI: 10.1140/epjs/s11734-022-00458-0