1. Neonatal Intensive Care and Epidemiological Unit, Department of Paediatrics, University of the Basque Country, Hospital de Cruces, Barakaldo-Bilbao, Spain.
2. Institute of Hygiene and Environmental Medicine, Charité-University Medicine Berlin, Germany.
3. Division of Neonatology and Paediatric Critical Care, Children's Hospital, University of Ulm, Germany.