(A, B, C) Daily swab positivity for all 19 rounds of the REACT-1 study (black points with 95% credible intervals, left hand y-axis) with P-spline estimates for swab positivity (solid black line, shaded area is 95% credible interval). Daily cases in England (green points, right hand y-axis) and P-spline model for expected daily cases in England (solid green line, shaded area is 95% credible interval, right hand y-axis). The 2 y-axes have been scaled using the population size and best-fit scaling parameter from the time-delay model fit to the rounds shown in each subfigure (available in S5 Table). (A) During round 1–7, daily cases have been shifted by 3 days backwards in time along the x-axis. (B) During round 8–13, daily cases have been shifted by 7 days forwards (−7 days backwards) in time along the x-axis. (C) During round 14–19, daily cases have been shifted by 1 day backwards in time along the x-axis. (D) Estimates of the case ascertainment rate over 19 rounds of REACT-1. Case ascertainment (solid black line, grey shaded region is 95% credible interval) estimated from the multiplicative difference between the REACT-1 P-spline for swab positivity and the time-delay adjusted P-spline for daily cases, accounting for population size, mean duration of positivity, and test sensitivity. The 95% credible intervals of the best-fitting average case ascertainment rates (red shaded area) over rounds 1–7, rounds 8–13, and rounds 14–19 estimated using separate time-delay models are shown for comparison (available in S5 Table). The black vertical dashed lines split the data into 3 periods: round 1–7, rounds 8–13, and rounds 14–19 of REACT-1. During rounds 1–7, the case ascertainment was estimated assuming a time-lag of 3 days. During rounds 8–13, the case ascertainment was estimated assuming a time-lag of −7 days. During rounds 14–19, the case ascertainment was estimated assuming a time-lag of 1 days. Data supporting this figure can be found in S4 Data.