Competing interests: The BMJ has judged that there are no disqualifying ties to commercial companies. The authors declare the following other interests: AD is the chief investigator on the CHELsea II trial study. SB is supported by the National Institute for Health and Care Research Applied Research Collaboration East of England (NIHR ARC EoE) at Cambridge and Peterborough NHS Foundation Trust. The views expressed are those of the authors and not necessarily those of the NIHR or the Department of Health and Social Care.