COVID-19 antibody testing study: a nested substudy within Alberta's Tomorrow Project (ATP) in Alberta, Canada

The Alberta's Tomorrow Project (ATP) prospective cohort study was established in 2000 to investigate the causes of cancer and chronic disease. The cohort consists of almost 55 000 participants aged 35-69 years at the time of recruitment. From 2020 to 2022, ATP conducted a longitudinal substudy, the COVID-19 Antibody Testing (CAT) study, nested in this existing cohort, to understand the spread and impact of the SARS-CoV-2. In this cohort profile, we describe the CAT study design, recruitment and initial findings.

In this prospective cohort substudy, ~4000 participants completed online surveys and provided blood samples at a study centre every 4 months for 1 year, across four cities in Alberta, Canada. The study was launched on a rolling basis beginning in September 2020 and data collection was completed in May 2022. The surveys collected information on health and lifestyle factors, COVID-19 (testing, symptoms, vaccination, public health recommendations) and impacts of the pandemic (including economic, health services, mental health). Blood samples were tested for antinucleocapsid and antispike protein SARS-CoV-2 antibodies.

A total of 4102 participants consented and attended a study centre at baseline, and almost 90% of these completed the study. Overall, participants were aged 61±10 years, 60% female, 12% came from rural areas, 45% had at least a bachelor's degree, 24% reported a household income <$C75 000 and 39% were retired. About 15% of participants tested positive for antibodies induced by a SARS-CoV-2 infection over the course of the study, and about 18% of those who were infected reported long COVID (persistent symptoms for >4 weeks). By the end of the study, 96% of participants had received at least one COVID-19 vaccine dose. Through investigating other outcomes, it was observed that participants under 50 years of age were more likely to be assessed to have mild or moderate-to-severe anxiety and depressive symptoms compared with older participants. In addition, approximately 15% of participants reported a moderate to major impact on their ability to meet financial obligations.

Serology results, together with health, lifestyle and sociodemographic data, and the continued follow-up of these participants as part of the broader ATP cohort study (planned through 2065), will provide opportunities to investigate the long-term sequelae of COVID-19 infection as well as the broader impacts of the pandemic on physical, mental and emotional health. Data are available to researchers on request through the ATP access process.