aHospital Epidemiology Service, Clinical Center, National Institutes of Health Service

bHospital Epidemiology Service, National Institutes of Health Clinical Center, Bethesda, Maryland, USA

Correspondence to David K. Henderson, MD, Senior Consultant, Hospital Epidemiology Service, National Institutes of Health Clinical Center, Room 9D46A 10 Center Drive, Bethesda, MD 20892, USA. Tel: +1 301 496 2209; e-mail: [email protected]