How did regional lockdowns during the COVID-19 pandemic affect recruitment into a large multinational cohort study of intrauterine device users?

Objectives: To investigate the impact of lockdown policies on the recruitment of an ongoing cohort study.

Study Design: We performed descriptive analyses of recruitment, dropout, and baseline characteristics over time. Oxford Stringency Index was used to assess the impact of regional constraints on recruitment.

Results: Drop in recruitment clearly reflected the Stringency Index within the first months of the pandemic. Unexpectedly, drop-out rates declined in 2020/2021. Baseline characteristics were comparable, yet younger women were recruited more frequently during the pandemic.

Conclusions: There was no strong evidence of recruitment bias due to the pandemic.

Implications: The COVID-19 pandemic is a potential source of bias for ongoing studies and its influence on the study conduct (e.g., recruitment, drop-out) should be thoroughly evaluated to ensure that study results are not biased in this regard. The Oxford's Government Stringency Index can be used to identify pandemic-affected time periods.