ACTB and infection: Notably, while SARS-CoV-2 replicated to a lower level in the lung of aged mice upon re-infection compared to the primary infection, with relative viral RdRp copy measured as 12.44/β-actin in the primary infection versus 0.44/β-actin in re-infected mouse lungs (p = 0.0309) (Figure 1(D) and 4(B)), Viral load in the NT of re-infected aged mice reached a similar level as those of the primary infection at 2 dpi, 1.32 vs. 1.27 RdRp copy/β-actin (Figure 1(C) and 4(B)).