Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. tjibbe.donker@rivm.nl|Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. tjibbe.donker@rivm.nl|Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. tjibbe.donker@rivm.nl