Understanding the treatment benefit of hyperimmune anti-influenza intravenous immunoglobulin (Flu-IVIG) for severe human influenza ‌ Hillary A. Vanderven, Deborah N. Wentworth, Win Min Han, Heidi Peck, Ian G. Barr, Richard T. Davey Jr., John H. Beigel, Dominic E. Dwyer, Mamta K. Jain, Brian Angus, Christian T. Brandt, Analia Mykietiuk, Matthew G. Law, James D. Neaton, Stephen J. Kent, for the INSIGHT FLU-IVIG Study Group