1. Division of Epidemiology and Biostatistics, Directorate-General of Health, Lisbon , Portugal
2. Division of Mother and Child Health, Directorate-General of Health, Lisbon, Portugal
3. Division of Virology, National Institute for Biological Standards and Control, London, UK
4. Division of Communicable Diseases, Directorate-General of Health, Lisbon, Portugal
5. Laboratory of Virology, National Institute of Health, Lisbon, Portugal