1 Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, Netherlands.
2 Department of Paediatrics, Spaarne Hospital, Hoofddorp, Netherlands.
3 University Groningen, University Medical Centre Groningen/Beatrix Children's Hospital, Groningen, Netherlands.
4 Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, Netherlands. Electronic address: nynke.rots@rivm.nl.