Virginia E Pitzer1, Andrew J Pollard2, Joke Bilcke3. 1. Department of Epidemiology of Microbial Diseases, Yale School of Public Health, Yale University, New Haven, CT, USA. 2. Oxford Vaccine Group, Department of Paediatrics, University of Oxford; NIHR Oxford Biomedical Research Centre, Oxford, UK. 3. Center for Health Economic Research and Modelling Infectious Diseases, Vaccine and Infectious Disease Institute, University of Antwerp, 2610 Antwerp, Belgium. Electronic address: joke.bilcke@uantwerp.be.