Enhanced Surveillance of COVID-19 in Scotland: population-based seroprevalence surveillance for SARS-CoV-2 during the first wave of the epidemic

Dickson, Elizabeth; Palmateer, Norah E.; Murray, Josie; Robertson, Chris; Waugh, Craig; Wallace, Lesley A.; Mathie, Lindsay; Heatlie, Karen; Mavin, Sally; Gousias, Petros; Von Wissman, Beatrix; Goldberg, David J.; McAuley, Andrew

Objectives. The impact of the COVID-19 pandemic in Scotland has been amongst the most severe in Europe. Serological surveillance is critical to determine the overall extent of infection across populations and to inform the public health response. This study aimed to estimate the proportion of people who have antibodies to SARS-CoV-2 (“seroprevalence”) in the general population of Scotland and to see if this changes over time. Study design/Methods. Between ISO week 17 (i.e. week commencing 20th April) and ISO week 25 (week commencing 15 June), 4751 residual blood samples were obtained from regional biochemistry laboratories in six participating regional health authority areas covering approximately 75% of the Scottish population. Samples were tested for the presence of anti-SARS-CoV-2 IgG antibodies using the LIAISON®SARS-CoV-2 S1/S2 IgG assay (DiaSorin, Italy). Seroprevalence rates were adjusted for the sensitivity and specificity of the assay using Bayesian methods. Results. The combined adjusted seroprevalence across the study period was 4.3% (95% CI 4.2%-4.5%). The proportion varied each week between 1.9% and 6.8% with no difference in antibody positivity by age, sex, or geographical area. Conclusions. At the end of the first wave of the COVID-19 pandemic, only a small fraction of the Scottish population had antibodies to SARS-CoV-2. Control of COVID-19 requires the ability to detect asymptomatic and mild infections, that would otherwise remain undetected through existing surveillance systems. This is important to determine the true number of infections within the general population which, in turn, can help to understand transmission, inform control measures, and provide a denominator for the estimation of severity measures such as the proportion of infected people who have been hospitalised and/or have died. The first COVID-19 diagnosis in Scotland was notified on 1 March 2020. WHO declared COVID-19 a global pandemic on 11 March 2020. On 23 March 2020, lockdown measures for Scotland and the rest of the UK were implemented. By the end of the first wave of the epidemic, Scotland was estimated to have the third highest rate of excess mortality in Europe after England and Spain [1]. Here, that sought to maximise the use of existing residual blood samples from primary care (general practice) settings to estimate the proportion of people who have antibodies to SARS-CoV-2 (“seroprevalence”) in the general population of Scotland and to see if this changes over time. The results presented here cover the pilot phase of the project between ISO week 17 and week 25.