The risks of attrition bias in longitudinal surveys of the impact of COVID-19.

As the COVID-19 pandemic continues, interest in mental health impacts is shifting from short-term to long-term outcomes. As part of a longitudinal online survey study examining mental health impacts of the pandemic, we assessed the risk of attrition bias related to a history of depression-a condition research shows can increase challenges of recruitment and retention. Among 5023 participants who completed the baseline survey, significantly more reporting a history of depression were lost to follow-up: baseline to 3 months: 497/760 (65.4%) vs 2228/4263 (52.3%),  < 0.001; 3 to 6 months: 179/263 (68.1%) vs 1183/2035 (58.1%),  = 0.002. Participants reporting a history of depression also had greater adjusted odds of a Patient Health Questionnaire-8 score ≥10 (odds ratio [OR] = 3.97, 95% confidence interval [CI] 3.27, 4.84), Generalized Anxiety Disorder-7 score ≥10 (OR = 3.77, 95% CI 3.07, 4.62), and Posttraumatic Diagnostic Scale for DSM V score ≥ 28 (OR = 7.17, 95% CI 4.67, 11.00) at baseline, indicating a need to account for attrition bias when examining these outcomes. Similar considerations likely apply to other longitudinal survey studies and are important to address to ensure accurate evidence is available to support policy decisions regarding resource allocation and funding.