Compared with the less virulent SARS-CoV-2 MA strain, the highly pathogenic MA20 variant caused a more widespread infection of the lung tissue as indicated by increased antigen-positive areas (Fig. S1 I and Fig. 3 F; about 10% mean antigen detection in SARS-CoV-2 MA infected adult Ifnar1−/− 3 d p.i., compared with about 67% mean antigen detection in MA20 infected adult WT mice 3 d p.i.). Here, IFNAR1 is linked to infection.