The prevalence of anti-SARS-CoV-2 antibodies and potential determinants were assessed in a random sample representative of the Belgian adult population. In total, 14,201 individuals (≥18 years) were invited by mail to provide saliva via an Oracol® swab. Survey weights were applied, and potential determinants were estimated using multivariable logistic regressions. Between March and August 2021, 2767 individuals participated in the first data collection. During this period, which coincided with the onset of the vaccination campaign, the seroprevalence in the population increased from 25.2% in March/April to 78.1% in July. Among the vaccinated there was an increase from 74,2% to 98.8%; among the unvaccinated, the seroprevalence remained stable (around 17%). Among the vaccinated, factors significantly associated with the presence of antibodies were: having at least one chronic disease (ORa 0.22 (95% CI 0.08–0.62)), having received an mRNA-type vaccine (ORa 5.38 (95% CI 1.72–16.80)), and having received an influenza vaccine in 2020–2021 (ORa 3.79 (95% CI 1.30–11.07)). Among the unvaccinated, having a non-O blood type (ORa 2.00 (95% CI 1.09–3.67)) and having one or more positive COVID-19 tests (ORa 11.04 (95% CI 4.69–26.02)) were significantly associated. This study provides a better understanding of vaccine- and/or natural-induced presence of anti-SARS-CoV-2 antibodies and factors that are associated with this presence.

Background This cohort study assessed seroprevalence trends of SARS-CoV-2 antibodies in the general Belgian population between March 2021 and April 2022, and explored factors associated with seropositivity among the vaccinated and unvaccinated population. Seroreversion and its potential determinants were also examined. Methods A random sample of the adult population in Belgium was invited to provide a saliva sample and to complete a survey questionnaire. Participants were followed up twice for a new saliva sample and updated information. Antibodies were assessed with a semi-quantitative measure of anti-RBD (Receptor Binding Domain) IgG ELISA. Seven time periods were defined for estimating SARS-CoV-2 antibody prevalence using post stratification weights to match the population distribution. Seroreversion was defined as passing from a positive to a negative antibody test from one data collection point to the next. Potential determinants of seropositivity were assessed through hierarchical multiple logistic regressions. Results In total 6,178 valid observations were obtained from 2,768 individuals. SARS-CoV-2 antibody prevalence increased from 25.1% in the beginning of the study period to 92.3% in the end. Among the vaccinated population, factors significantly associated with a higher seropositivity were being younger, having a bachelor diploma, living with others, having had a vaccine in the last 3 months and having received a nucleic-acid vaccine or a combination. Lower seropositivity rates were observed among vaccinated people with a neurological disease and transplant patients. Factors significantly associated with a higher seropositivity rate among the unvaccinated population were having non-O blood type and being non-smoker. Among fully vaccinated people the seroreversion rate was much lower (0.3%) among those who had received their latest vaccine in the last 3 months compared to those who had received their latest vaccine more than 3 months ago (2.7%). Conclusions The rapid increase in antibody seropositivity in the general adult population in Belgium during the study period was driven by the vaccination campaign which ran at full speed during this period. Factors associated with higher and lower seropositivity were identified among the vaccinated and unvaccinated people.