Fig. 3. Lung viral titers in naïve mice receiving passive serum transfer from young and aged immunized mice upon challenge with MA-SARS-CoV-2 and B.1.351.

a Sera from young or aged mice given two IN immunizations with 20 μg RBD with PBS, NE, or NE/IVT were pooled at week 6, and 50 μL of the pooled serum was transferred IP into each naïve mouse 2 h prior to challenge IN with 10⁴ pfu MA-SARS-CoV-2. b Sera from young or aged mice given three IN immunizations with 20 μg RBD with PBS, NE, or NE/IVT were pooled at week 10, and 110 μL of the pooled serum was transferred IP into each naïve mouse 2 h prior to challenge IN with 5 × 10³ pfu B.1.351. Viral load was assessed in lung homogenate by plaque assay at 3 days post-infection (d.p.i.). The plaque detection limit is indicated by the horizontal dashed line. Lung viral titers are shown as mean ± SEM (n = 5/grp) (*p < 0.05, **p < 0.01 by Mann–Whitney U test).