Randomisation and masking
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Participants were randomly assigned (1:1) using a remote, secure web-based clinical research system (P1vital® ePRO) to either immediate intervention arm (immediate access to the brief digital imagery-competing task intervention plus symptom monitoring for 4 weeks) or delayed intervention arm (usual care for 4 weeks followed by access to the intervention plus symptom monitoring for 4 weeks).Randomisation occurred following the baseline week, and after baseline questionnaires (see protocol; osf.io/2xn5m). The allocation sequence was computer-generated consecutively using minimisation. Minimisation allocated the first participant to either arm, thereafter allocation was preferential to the arm with the fewest participants to minimise the difference in group sizes. The randomisation allocation percentage was originally set to 66% and altered to 85% after 61 participants were randomised (to ensure balance of groups after early stopping decision, see “Results”). Overall, 86 participants were randomly assigned to the delayed (Participants were blinded to group allocation. The statistician who conducted the interim analyses (VR), and researchers who contacted participants and facilitated the conduct of the task intervention were not blinded to group allocation. A researcher supported the technical use of the intervention in the first guided session in i-spero® and was aware that the participant completed a cognitive task. It is noted that all outcome data (including the primary outcome 4 weeks later) was self-reported by participants independently and directly into the online platform (P1vital® ePRO) removing the need for researcher input and reducing the risk of bias. That is, outcome assessments were masked to group allocation since they were self-reported by participants in the digital platform. Scoring of data was automatic in P1vital® ePRO, and thus independent from the researchers guiding the intervention session.