BackgroundIn 2020, Wales experienced some of the highest rates of confirmed COVID-19 cases in Europe. We set up a serosurveillance scheme using residual samples from blood donations to inform the pandemic response in Wales.AimTo identify changes in SARS-CoV-2 antibody seroprevalence in Wales by time, demography and location.MethodsResidual samples from blood donations made in Wales between 29 June 2020 and 20 November 2022 were tested for antibodies to the nucleocapsid antigen (anti-N) of SARS-CoV-2, resulting from natural infection. Donations made between 12 April 2021 and 20 November 2022 were also tested for antibodies to the spike antigen (anti-S) occurring as a result of natural infection and vaccination.ResultsAge-standardised seroprevalence of anti-N antibodies in donors remained stable (4.4–5.5%) until November 2020 before increasing to 16.7% by February 2021. Trends remained steady until November 2021 before increasing, peaking in November 2022 (80.2%). For anti-S, seroprevalence increased from 67.1% to 98.6% between May and September 2021, then remained above 99%. Anti-N seroprevalence was highest in younger donors and in donors living in urban South Wales. In contrast, seroprevalence of anti-S was highest in older donors and was similar across regions. No significant difference was observed by sex. Seroprevalence of anti-N antibodies was higher in Black, Asian and other minority ethnicities (self-reported) compared with White donors, with the converse observed for anti-S antibodies.ConclusionWe successfully set up long-term serological surveillance of SARS-CoV-2 using residual samples from blood donations, demonstrating variation based on age, ethnicity and location.