KRT18 and infection: To examine the SΔ19 Rep infectivity and the duration of S protein expression in mice, the K18-hACE2 transgenic mice were infected with the SΔ19 Rep R682G-S813Y virus (1x108 pfu/mouse; i.n.), and the expression of S protein in lung collected from Day 1 to 4 post-infection was monitored by immunostaining (Figure S3).