1. Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College, Dublin 2, Ireland1
2. Department of Veterinary Pathology, Faculty of Veterinary Medicine, University College Dublin, Dublin 4, Ireland2
3. Department of Vaccine Research, Swedish Institute for Infectious Disease Control, S-171 82 Solna, Sweden4
4. Microbiology and Tumorbiology Center, Karolinska Institute, S-171 77 Stockholm, Sweden3