1. Emory University School of Medicine, 1510 Clifton Rd., Atlanta, Georgia 30322
2. Influenza Branch MS G-16, Centers for Disease Control, 1600 Clifton Rd., Atlanta, Georgia, 30333
3. Division of Virology, Nation Institute for Biological Standards and Controls, Blanche Lane, Potters Bar, Hertfordshire EN6 3QG, United Kingdom
4. National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom
5. School of Biology, University of St. Andrews, Fife KY16 9ST, United Kingdom