Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. PRH is funded by the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Emergency Preparedness and Response at King’s College London in partnership with Public Health England (PHE) and collaboration with the University of East Anglia. The views expressed are those of the author and not necessarily those of the NHS, the NIHR, UEA, the Department of Health, or PHE. The BMJ policy on financial interests is here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf. Provenance and peer review: Commissioned; not externally peer reviewed.