Daniel A Salmon1, C Raina MacIntyre2, Saad B Omer3. 1. Johns Hopkins School of Public Health, Baltimore, MD 21209, USA. Electronic address: dsalmon1@jhu.edu. 2. School of Public Health and Community Medicine, University of New South Wales, Kensington, NSW, Australia. 3. Rollins School of Public Health, Emory University, Schools of Public Health & Medicine and Emory Vaccine Center, Atlanta, GA, USA.