- Correspondence to: Helen Ward Department of Infectious Disease Epidemiology, Imperial College London, Norfolk Place, London W2 1PG, UK;
- Accepted February 24, 2007
- First published March 27, 2007.
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.