title: In reply.
authors.affiliation: University College London, UK.
authors.forename: Helen R
authors.initials: HR
authors.lastname: Stagg
authors.affiliation: Public Health England, UK.
authors.forename: Ross J
authors.initials: RJ
authors.lastname: Harris
authors.affiliation: University College London, Royal Free London National Health Service Foundation Trust, London, UK. h.stagg@ucl.ac.uk.
authors.forename: Marc C
authors.initials: MC
authors.lastname: Lipman