Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The authors declare the following other interests: AM is the principal investigator on an unrelated study sponsored by Astra Zeneca. Further details of The BMJ policy on financial interests are here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf. Provenance and peer review: Not commissioned; externally peer reviewed.