Challenges in control of Covid-19: short doubling time and long delay to effect of interventions by Pellis, L, Scarabel, F, Stage, HB, Overton, CE, Chappell, LHK, Lythgoe, KA, Fearon, E, Bennett, E, Curran-Sebastian, J, Das, R, Fyles, M, Lewkowicz, H, Pang, X, Vekaria, B, Webb, L, House, TA, Hall, I