Introduction COVID-19 is an unprecedented public health threat in modern times, especially for older adults or those with chronic illness. Beyond the threat of infection, the pandemic may also have longer-term impacts on mental and physical health. The COVID-19 & Chronic Conditions (‘C3’) study offers a unique opportunity to assess psychosocial and health/healthcare trajectories over 5 years among a diverse cohort of adults with comorbidities well-characterised from before the pandemic, at its onset, through multiple surges, vaccine rollouts and through the gradual easing of restrictions as society slowly returns to ‘normal’. Methods and analysis The C3 study is an extension of an ongoing longitudinal cohort study of ‘high-risk’ adults (aged 23–88 at baseline) with one or more chronic medical conditions during the COVID-19 pandemic. Five active studies with uniform data collection prior to COVID-19 were leveraged to establish the C3 cohort; 673 adults in Chicago were interviewed during the first week of the outbreak. The C3 cohort has since expanded to include 1044 participants across eight survey waves (T1–T8). Four additional survey waves (T9–T12) will be conducted via telephone interviews spaced 1 year apart and supplemented by electronic health record and pharmacy fill data, for a total of 5 years of data post pandemic onset. Measurement will include COVID-19-related attitudes/ behaviours, mental health, social behaviour, lifestyle/ health behaviours, healthcare use, chronic disease self-management and health outcomes. Mental health trajectories and associations with health behaviours/ outcomes will be examined in a series of latent group and mixed effects modelling, while also examining mediating and moderating factors. Ethics and dissemination This study was approved by Northwestern University’s Feinberg School of Medicine Institutional Review Board (STU00215360). Results will be published in international peer-reviewed journals and summaries will be provided to the funders of the study.
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