Detecting changes in population trends in infection surveillance using community SARS-CoV-2 prevalence as an exemplar by Pritchard, E, Vihta, K-D, Eyre, DW, Hopkins, S, Peto, TEA, Matthews, PC, Stoesser, N, Studley, R, Rourke, E, Diamond, I, Pouwels, KB, Walker, AS, COVID-19 Infection Survey Team