Protocol of the process evaluation of cluster randomised control trial for estimating the effectiveness and cost-effectiveness of a complex intervention to increase care home staff influenza vaccination rates compared to usual practice (FluCare).

Background: Influenza (flu) vaccination rates in UK care home staff are extremely low. Less than 40% of staff in care homes are vaccinated for influenza (flu), presenting risks to the health of frail residents and potential staff absence from cross-infection. Staff often do not perceive a need for vaccination and are unaware they are entitled to free flu vaccination. The FluCare study, a cluster randomised control trial (RCT), uses behavioural interventions to address barriers. Videos, posters, and leaflets are intended to raise awareness of flu vaccination benefits and debunk myths. On-site staff vaccination clinics increase accessibility. Financial incentives to care homes for improved vaccination rates and regular monitoring influence the environment. This paper outlines the planned process evaluation which will describe the intervention's mechanisms of action, explain any changes in outcomes, identify local adaptations, and inform design of the implementation phase.

Methods/design: A mixed method process evaluation to inform the interpretation of trial findings.

Objectives: • Describe the intervention as delivered in terms of dose and fidelity, including adaptations and variations across care homes. • Explore the effects of individual intervention components on primary outcomes. • Investigate the mechanisms of impact. • Describe the perceived effectiveness of relevant intervention components (including videos, leaflets, posters, and flu clinics) from participant perspectives (care home manager, care home staff, flu clinic providers). • Describe the characteristics of care homes and participants to assess reach. A purposive sample of twenty care homes (ten in the intervention arm, ten in the control arm) for inclusion in the process evaluation. Data will include (1) study records including care home site profiles, (2) responses to a mechanism of action questionnaire, and (3) semi-structured interviews with care home staff and clinic providers. Quantitative data will be descriptively reported. Interview data will be thematically analysed and then categories mapped to the Theoretical Domains Framework.

Discussion: Adopting this systematic and comprehensive process evaluation approach will help ensure data is captured on all aspects of the trial, enabling a full understanding of the intervention implementation and RCT findings.

Trial Registration: ISRCTN ISRCTN22729870. Registered on 24 August 2022.