COVID-19 risk mitigation in reopening mass cultural events: population-based observational study for the UK Events Research Programme in Liverpool City Region Bằng Burnside, G, Cheyne, CP, Leeming, G, Humann, M, Darby, A, Green, MA, Crozier, A, Maskell, S, O'Halloran, K, Musi, E, Carmi, E, Khan, N, Fisher, D, Corcoran, R, Dunning, J, Edmunds, WJ, Tharmaratnam, K, Hughes, DM, Malki-Epshtein, L, Cook, M, Roberts, BM, Gallagher, E, Howell, K, Chand, M, Kemp, R, Boulter, M, Fowler, T, Semple, MG, Coffey, E, Ashton, M, García-Fiñana, M, Buchan, IE