William P Hanage1, Colin A Russell2. 1. Department of Epidemiology, Harvard University T. H. Chan School of Public Health, Boston, MA 02115, USA. w.hanage@hsph.harvard.edu c.a.russell@amsterdamumc.nl. 2. Department of Medical Microbiology, Amsterdam University Medical Center, Amsterdam, Netherlands. w.hanage@hsph.harvard.edu c.a.russell@amsterdamumc.nl.