The C19PRC study is a large, longitudinal online survey of the general adult population aged 18 years and older during the COVID-19 pandemic. The study involved regular and timely repeated assessments as the pandemic unfolded. Although the core C19PRC Study team is based in the United Kingdom, existing collaborations were leveraged to establish similar studies in the Republic of Ireland, Spain, and Italy. The C19PRC-UK survey launched on 23 March 2020, the day that a strict lockdown was enforced across the UK, and studies in the other European countries were initiated shortly thereafter.