Acknowledgements
We thank Prof. Dr. Timm Harder and Prof. Dr. Martin Beer, Friedrich-Loeffler-Institute, Greifswald - Isle of Riems, Germany for establishment of ferret antisera; Mareen Adam, Susi Hafemann, Ute Hopf-Guevara, Carmen Karstädt-Schulze, Katja-Irena Madaj, Jeanette Milde, Bettina Mischke, Christine Spingies, Anneliese Schindel, Maria Smallfield and Nathalie Tollard for excellent technical assistance; Heike Fischer and Kathrin Seidel for outstanding sequencing support.
We thank Dr. John McCauley and Áine Rattigen, WHO Collaborating Centre, Francis Crick Institute, London for providing vaccine viruses and antisera.