Cohort retention in a pandemic response study: Lessons from the SARS-Cov-2 Immunity & Reinfection Evaluation (SIREN) study

Abstract Background The SIREN Study is a healthcare worker cohort study aiming to determine COVID-19 incidence, duration of immunity and vaccine effectiveness across 135 NHS organisations in four UK nations. Conducting an intensive prospective cohort study during a pandemic was challenging. We designed an evolving retention programme, utilising participant engagement tools underpinned by our Participant Involvement Panel (PIP) and here we evaluate cohort retention over time and identify learnings. Methods A mixed method evaluation of cohort retention in 12 and 24-month follow-up. We described cohort retention by demographics and site, using odds ratios from logistic regression. Withdrawal reasons were collected by survey. We collected participant feedback via cross-sectional online survey utilising a behavioural science approach. We conducted focus groups with site teams and conducted thematic analysis exploring cohort retention challenges and facilitators. Results 37,275 (84.7%) participants completed 12-months of follow-up. Of 14,772 participants extending their follow-up to 24 months, 12,635 (85.5%) completed this. Retention increased with age in the 12 (55–64 years vs < 25 years OR = 2.50; 95% CI: 2.19–2.85) and 24-month (> 65 years vs < 25 years OR = 2.92; 95% CI: 1.78–4.88) cohorts. Retention was highest in the Asian and Black compared to White ethnic groups in the 12 (OR = 1.38; 95% CI: 1.23–1.56; p < 0.001, and OR = 1.64; 95% CI: 1.30–2.08) and 24-month (OR = 1.78; 95% CI: 1.42–2.25; p < 0.001, and OR = 2.12; 95% CI: 1.41–3.35) cohort. Among participants withdrawing, the median time in follow-up at withdrawal was 7 months (IQR: 4–10 months) within the 12-month cohort and 19 months within the 24-month cohort (IQR: 16–22 months). The top three reasons for participant withdrawal were workload, leaving site employment and medical reasons. Themes identified from focus-groups included: the need to monitor and understand participant motivation over time, the necessity of inclusive and comprehensive communication, the importance of acknowledging participant contributions, building collaboration with local research teams, and investing in the research team skillset. Conclusion Participant retention in the SIREN study remained high over 24-months of intensive follow-up, demonstrating that large-cohort studies are feasible as a pandemic research tool. Insights into facilitators and challenges provide learning for future studies.