PROS1 and infection: Alpha-2,3-linked sialic acids have been implicated to be essential for the attachment and infection of IBV (Wickramasinghe et al., 2011; Promkuntod et al., 2014), and N-glycosylation of IBV S protein is a determinant for receptor-binding specificity (Parsons et al., 2019; Bouwman et al., 2020a,b).