1. Department of Microbiological Surveillance and Research, Statens Serum Institut, 5 Artillerivej, DK-2300 Copenhagen S, Denmark
2. Centre for Infection, Department of Cellular & Molecular Medicine, St George's, University of London, Cranmer Terrace, London SW17 0RE, United Kingdom
3. Department of Disease Biology, Faculty of Life Sciences, University of Copenhagen, 4 Stigbøjlen, DK1870 Frederiksberg C, Denmark
4. Quantitative Veterinary Epidemiology Group, Wageningen Institute of Animal Sciences, Wageningen University, P.O. Box 338, 6700 AH Wageningen, Netherlands
5. Centre for Infectious Disease Control Netherlands, National Institute for Public Health and the Environment, P.O. Box 1, 3720BA, Bilthoven, Netherlands