(a-b) OM Mal/04 Influenza B viral titers (a) and plasma nAb titers (b) from IN infected mice. Experimental groups: mice infected with Mal/04 21d prior (n = 5), mice receiving anti-Mal/04 convalescent plasma (n = 5) 12h before challenge, and naïve mice (n = 5). Data representative of two independent experiments. (c-d) mT/mG mice were IN infected with Mal/04-Cre and fixed 48h later for immunofluorescent confocal microscopy. Mice received either no treatment (n = 4) or convalescent plasma transfer (n = 4) 12h prior to challenge. Representative images of OE and RE tissue shown in (c). Quantification of infected cells (GFP+) per mm of epithelium, 3 sagittal sections per mouse (d). Scale bars = 150 μm. Statistics in (a) calculated by ordinary One-Way ANOVA with multiple comparisons. Parametric unpaired t-test performed in (b). Comparisons in (d) evaluated using unpaired t-test with Welch’s correction, ns (not significant) P > 0.05, * P < 0.05, ** P < 0.01, *** P < 0.001, **** P < 0.0001.