AbstractBackgroundIn the first weeks of the COVID-19 epidemic in Belgium, a repetitive national serum collection was set up to monitor age-related exposure through emerging SARS-CoV-2 antibodies. First objective was to estimate the baseline seroprevalence and seroincidence using serial survey data that covered the start of a national lock-down period installed soon after the epidemic was recognized.MethodsA prospective serial cross-sectional seroprevalence study, stratified by age, sex and region, started with two collections in April 2020. In residual sera taken outside hospitals and collected by diagnostic laboratories, IgG antibodies against S1 proteins of SARS-CoV-2 were measured with a semi-quantitative commercial ELISA. Seropositivity (cumulative, by age category and sex) and seroincidence over a 3 weeks period were estimated for the Belgian population.FindingsIn the first collection, IgG antibodies were detected in 100 out of 3910 samples, whereas in the second collection 193 out of 3391 samples were IgG positive. The weighted overall seroprevalence increased from 2·9% (95% CI 2·3 to 3·6) to 6·0% (95% CI 5·1 to 7·1), reflected in a seroincidence estimate of 3·1% (95% CI 1·9 to 4·3). Age-specific seroprevalence significantly increased in the age categories 20-30, 80-90 and ≥90. No significant sex effect was observed.InterpretationDuring the start of epidemic mitigation by lockdown, a small but increasing fraction of the Belgian population showed serologically detectable signs of exposure to SARS-CoV-2.FundingThis independent researcher-initiated study acknowledges financial support from the Antwerp University Fund, the Flemish Research Fund, and European Horizon 2020.Research in contextEvidence before this studyThis is the first study reporting seroprevalence and seroincidence of IgG against SARS-CoV-2 in the Belgian population. Worldwide, PCR tests are being performed to identify mainly sick people suffering from COVID-19. However, seroprevalence studies are important and feasible to study the proportion of the population that has already been in contact with the virus, which helps to understand the likelihood of asymptomatic infections or infections with mild symptoms.From 11 March to 11 May, updates on the COVID-19 pandemic by the World Health Organisation as well as bulletins from the Belgian Scientific Institute for Public Health, Sciensano, were consulted daily. Press releases from all over the world were monitored during that period. Google, PubMed as well as the pre-print server medrxiv were consulted by searching the terms “seroprevalence SARS-CoV-2” and “COVID-19”,Added value of this studyThis study reports that seroprevalence increased in Belgium from 2·9% (95% CI 2·3 to 3·6) to 6·0% (95% CI 5·1 to 7·1) over a period of 3 weeks during lockdown (30 March-5 April 2020 & 20-26 April 2020) with seroincidence estimate of 3·1% (95% CI 1·9 to 4·3). Moreover, a significant increase in seroprevalence in the age categories 20-30 and ≥80 and within each sex were reported.Implications of all the available evidenceSeroprevalences worldwide indicate that an increasing fraction of the population has already been exposed to SARS-CoV-2. The continuous monitoring of seroprevalences is valuable to calibrate the response to the epidemic and to guide policy makers to control the epidemic wave and potential future waves and to avoid a deconfinement strategy leading to a rebound. However, it seems likely that natural exposure during this pandemic might not soon deliver the required level of herd immunity and there will be a substantial need for mass vaccination programmes to save time and lives.
Introduction
