1. Department of Medical Microbiology, Section Clinical Virology, University Medical Center Groningen, P.O. Box 30.001, 9700 RB Groningen, The Netherlands
2. Department of Medical Microbiology, Section Clinical Virology, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands
3. Department of Pediatrics, Leiden University Medical Center, Leiden, The Netherlands
4. Department of Pediatrics, University Medical Center Groningen, Groningen, The Netherlands