Digital measurement of SARS-CoV-2 transmission risk from 7 million contacts by Ferretti, L, Wymant, C, Petrie, J, Tsallis, D, Kendall, M, Ledda, A, Di Lauro, F, Fowler, A, Di Francia, A, Abeler-Dörner, L, Charalambides, M, Briers, M, Fraser, C