- Correspondence to Dr MAB van der Sande, Epidemiology and Surveillance Unit, Centre for Infectious Disease Control, National Institute for Public Health and the Environment, PO Box 1—pb 75, 3720 BA Bilthoven, The Netherlands;
- Received December 10, 2014
- Revised March 27, 2015
- Accepted April 21, 2015
- First published May 11, 2015.
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