Publication date: Jan 30, 2025

SIREN 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, informed by emerging evidence on best practice. This included applying a multifactorial approach, and considering strategies for barrier reduction, community building, follow-up, and tracing. We utilised participant engagement tools underpinned by our Participant Involvement Panel (PIP) and here we evaluate cohort retention over time and identify learnings. A mixed method evaluation of cohort retention in 12 and 24-month follow-up (June 2020 – March 2023). We described cohort retention by demographics and site, using odds ratios from logistic regression. Withdrawal reasons during this time were collected by survey. We collected participant feedback via cross-sectional online survey conducted October – November 2022, utilising a behavioural science approach. We conducted two focus groups with research teams in February 2023 and conducted thematic analysis exploring cohort retention challenges and facilitators. 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  65 years vs 

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