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Background: Most adults in the UK experience at least one viral respiratory tract infection (RTI) per year. Individuals with comorbidities and those with recurrent RTIs are at higher risk of infections. This can lead to more severe illness, worse quality of life and more days off work. There is promising evidence that using common nasal sprays or improving immune function through increasing physical activity and managing stress, may reduce the incidence and severity of RTIs. Methods and design: Immune Defence is an open, parallel group, randomised controlled trial. Up to 15000 adults from UK general practices, with a comorbidity or risk factor for infection and/or recurrent infections (3 or more infections per year) will be randomly allocated to i) a gel-based nasal spray designed to inhibit viral respiratory infections; ii) a saline nasal spray, iii) a digital intervention promoting physical activity and stress management, or iv) usual care with brief advice for managing infections, for 12 months. Participants will complete monthly questionnaires online. The primary outcome is the total number of days of illness due to RTIs over 6 months. Key secondary outcomes include: days with symptoms moderately bad or worse; days where work/normal activities were impaired; incidence of RTI; incidence of COVID-19; health service contacts; antibiotic usage; beliefs about antibiotics; intention to consult; number of days of illness in total due to respiratory tract infections over 12 months. Economic evaluation from an NHS perspective will compare the interventions, expressed as incremental cost effectiveness ratios. A nested mixed methods process evaluation will examine uptake and engagement with the interventions and trial procedures. Trial status: Recruitment commenced in December 2020 and the last participant is expected to complete the trial in April 2024. Discussion: Common nasal sprays and digital interventions to promote physical activity and stress management are low cost, accessible interventions applicable to primary care. If effective, they have the potential to reduce the individual and societal impact of RTIs. Trial registration: Prospectively registered with ISRCTN registry (17936080) on 30/10/2020. Sponsor: This RCT is sponsored by University of Southampton. The sponsors had no role in the study design, decision to publish, or preparation of the manuscript. [ABSTRACT FROM AUTHOR]