Sampling strategy
RECRUITMENT
Participants enrolled in the SAFA trial were invited to take part in an optional qualitative interview with no monetary incentive. Participants were invited to take part opportunistically (based on their willingness and ability) by research staff at sites or by unblinding letters at 24 weeks. The total number of invited participants by research sites is unknown because they were not requested to keep a log, and 174 were invited through an unblinding letter sent 28 weeks after their baseline visit. Trial participants who were interested in being interviewed were asked to email the SAFA qualitative study mailbox accessed by the researchers conducting interviews (CB, AS, CC), who then responded to them via email to provide the Qualitative Research Information Sheet and Qualitative Interview Consent Form. There was no decline slip, therefore the reasons for not taking part are unknown. Once a hand or electronically signed consent was received, the interview time was confirmed and subsequently conducted by telephone or video call (participant preference). Participants were asked if they had questions during the recruitment process and again immediately before the interview.The qualitative sub-study was not funded as part of the trial and resources and ethical approval for this only later became available. Trial recruitment ran from June 2019 to August 2021, whilst invitations for qualitative interviews started in August 2021.