National population prevalence of antibodies to SARS-CoV-2 in Scotland during the first and second waves of the COVID-19 pandemic

Palmateer, Norah E.; Dickson, Elizabeth; Furrie, Elizabeth; Godber, Ian; Goldberg, David J.; Gousias, Petros; Jarvis, Lisa; Mathie, Lindsay; Mavin, Sally; McMenamin, Jim; McNeilly, Tom N.; Murcia, Pablo; Murray, Josie; Reid, Graeme; Robertson, Chris; Templeton, Kate; von Wissmann, Beatrix; Wallace, Lesley A.; Waugh, Craig; McAuley, Andrew

Objectives Studies that measure the prevalence of antibodies to SARS-CoV-2 (“seroprevalence”) are essential to understand population exposure to SARS-CoV-2 among symptomatic and asymptomatic individuals. We aimed to measure seroprevalence in the Scottish population over the course of the COVID-19 pandemic – from prior to the first recorded case in Scotland, through to the second pandemic wave. Study design Serial cross-sectional. Methods We tested 41,477 residual samples retrieved from primary and antenatal care settings across Scotland for SARS-CoV-2 antibodies over a 12-month period from December 2019-December 2020 (pre-rollout of COVID-19 vaccination). Five-weekly rolling seroprevalence estimates were adjusted for the sensitivity and specificity of the assays and weighted to reference populations. Temporal trends in seroprevalence estimates and weekly SARS-CoV-2 PCR positive notifications were compared. Results Five-weekly rolling seroprevalence rates were 0% until the end of March, when they increased contemporaneously with the first pandemic wave. Seroprevalence rates remained stable through the summer (range 3%–5%) during a period of social restrictions, following which they increased concurrently with the second wave, reaching 9.6% (95% CI 8.4%-10.8%) in week beginning 28th December 2020. Seroprevalence rates were lower in rural vs. urban areas (adjusted odds ratio[AOR] 0.70, 95% CI 0.61–0.79) and among individuals aged 20-39 and 60+ (AORs 0.74, 95% CI 0.64–0.86, and 0.80, 95% CI 0.69–0.91, respectively) relative to those aged 0-19 years. Conclusions After two waves of the COVID-19 pandemic, less than one in ten individuals in the Scottish population had antibodies to SARS-CoV-2. Seroprevalence may underestimate the true population exposure as a result of waning antibodies among individuals who were infected early in the first wave.