1. Infectious Diseases Unit, Department of Medicine, Karolinska Institute, Karolinska University Hospital, Solna, S-171 76 Stockholm, Sweden.
2. Clinical Epidemiology Unit, Department of Medicine, Karolinska Institute, Karolinska University Hospital, Solna, S-171 76 Stockholm, Sweden.
3. Division of Clinical Virology, Department of Immunology, Microbiology and Pathology (IMPI), Huddinge University Hospital, S-141 86, Stockholm, Sweden.
4. Present address: Department of Virology, Swedish Institute for Infectious Disease Control (SMI), S-171 82, Solna, Sweden.
5. Infectious Diseases Unit, Department of Medicine, Karolinska Institute, Karolinska University Hospital, Huddinge, S-141 86 Stockholm, Sweden.