Clade Ib SARS-CoV-2 MA10 infection in CCR9−/− mice also developed more severe clinical disease including greater weight loss (P < 0.0001 to P < 0.05) (Fig. 6A) and virus titers (P < 0.0005) (Fig. 6B) than C57BL/6NJ mice. Here, CCR9 is linked to infection.