WFDC2 and COVID-19: Our study showed that four key proteins, WFDC2 (WAP four-disulfide core domain protein 2, also known as human epididymis protein 4 [HE4]) [26, 27], GDF-15 (growth differentiation factor 15) [28], CHI3L1 (chitinase-3 like-protein-1, also known as YKL-40) [29], and KRT19 (keratin, type I cytoskeletal 19) [30], were associated with the prognosis of COVID-19, and this is supported by the previous reports.