Efficacy of passive immunotherapy of influenza A virus pneumonia by i.v. injection of IVIG at doses of 100, 500, 1000 and 5000 μg/mouse (a) or intranasal administration of IVIG at doses of 1, 10, 25, 50 μg/mouse (b) at 3 h after intranasal challenge with the influenza A strain Scotland. Data are the means of two independent experiments, for each route of IVIG administration, using each lot of IVIG, with 10 mice per group in each experiment.