Vitamin D insufficiency in COVID-19 and influenza A, and critical illness survivors: a cross-sectional study by Hurst, EA, Mellanby, RJ, Handel, I, Griffith, DM, Rossi, AG, Walsh, TS, Shankar-Hari, M, Dunning, J, Homer, NZ, Denham, SG, Devine, K, Holloway, PA, Moore, SC, Thwaites, RS, Samanta, RJ, Summers, C, Hardwick, HE, Oosthuyzen, W, Turtle, L, Semple, MG, Openshaw, PJM, Baillie, JK, Russell, CD