Background Migrants to the UK face disproportionate risk of infections, non-communicable diseases, and under-immunisation compounded by healthcare access barriers. Current UK migrant screening strategies are unstandardised with poor implementation and low uptake. Health Catch-UP! is a collaboratively produced digital clinical decision support system that applies current guidelines (UKHSA and NICE) to provide primary care professionals with individualised multi-disease screening (7 infectious diseases/blood-borne viruses, 3 chronic parasitic infections, 3 non-communicable disease or risk factors) and catch-up vaccination prompts for migrant patients, which needs evaluating as a complex intervention to explore effectiveness and acceptability. Methods We carried out a mixed-methods process evaluation of Health Catch-UP! in two urban primary healthcare practices to integrate Health Catch-UP! into the electronic health record system of primary care, using the Medical Research Council framework for complex intervention evaluation. We collected quantitative data (demographics, patients screened, disease detection and catch-up vaccination rates) and qualitative participant interviews to explore acceptability and feasibility. Results 99 migrants were assessed by Health Catch-UP! across two sites (S1, S2). 96.0 % (n=97) had complete demographics coding with Asia 31.3 % (n= 31) and Africa 25.2% (n=25) the most common continents of birth (S1 n=92 [48.9% female (n=44); mean age 60.6 years (SD 14.26)]; and S2 n=7 [85.7% male (n=6); mean age 39.4 years (SD16.97)]. 61.6% (n=61) of participants were eligible for screening for at least one condition and uptake of screening was high 86.9% (n= 53). Twelve new conditions were identified (12.1% of study population) including hepatitis C (n=1), hypercholesteraemia (n= 6), pre-diabetes (n=4) and diabetes (n=1). Health Catch-UP! identified that 100% (n=99) of patients had no immunisations recorded; however, subsequent catch-up vaccination uptake was poor (2.0%, n=1). Qualitative data supported acceptability and feasibility of Health Catch-UP! from staff and patient perspectives, and recommended Health Catch-UP! integration into routine care (e.g. NHS health checks) but required an implementation package including staff and patient support materials, standardised care pathways (screening and catch-up vaccination, laboratory, and management), and financial incentivisation. Conclusions Clinical Decision Support Systems like Health Catch-UP! can improve disease detection and implementation of screening guidance for migrant patients but require robust testing, resourcing, and an effective implementation package to support both patients and staff.

FW is a member of the Vulnerable Migrants Wellbeing Project Advisory Board, led by the University of Birmingham and Doctors of the World and funded by the Nuffield Foundation. All other authors declare they have no competing interests. The views expressed are those of the author(s) and not necessarily those of the NHS, Department of Health and Social Care, or the NIHR.

This study was funded by the Academy of Medical Sciences (SBF005I1) through a Springboard Award to SH. JC is funded by the NIHR and the Wellcome Trust. FK is funded by the NIHR. SH is additionally funded by the NIHR (NIHR300072; NIHR134801), Academy of Medical Sciences (SBF005I1) the La Caixa Foundation (LCF/PR/SP21/52930003), Research England, MRC, and WHO. AM is supported by the NIHR Applied Research Collaboration NW London.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

This study received ethics approval from the Health Research Authority and Health and Care Research Wales (IRAS 290630 reference 21/LO/0299), St George's, University of London Research Ethics Committee (2020.00630) and the Health Research Authority (REC 20/HRA/1674).

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors