1. Department of Pediatrics, Dartmouth-Hitchcock Medical Center, One Medical Center Drive, Lebanon, NH 03756, USA
2. Department of Epidemiology, Geisel School of Medicine, Dartmouth College, Hanover, NH, USA
3. Department of Pediatrics, Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA
4. Bowdoin College, Brunswick, ME, USA
5. Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK