ESTIMATING THE INCIDENCE OF SARS-COV-2 INFECTIONS IN 2020 IN BELGIUM BY JOINTLY MODELLING SEROPREVALENCE, HOSPITALIZATION AND MORTALITY DATA

1.1. Background The incidence of SARS-CoV-2 infections was not directly observed during the pandemic. We estimated incidences and the associated burden, through the infection fatality rate (IFR) and infection hospitalization rate (IHR), by jointly modeling seroprevalence, hospitalization, and mortality data.   1.2. Methods We used a hierarchical Bayesian model to fit a spline, representing incidence, to the observed number of hospitalizations, deaths and seroprevalence results. Two prospective cross-sectional seroprevalence studies on the prevalence of SARS-CoV-2 antibodies in the general population collected 37,235 samples in the age interval 18 to 74 years across 30 time points between March 2020 and January 2021. One study analyzed residual laboratory samples with the Euroimmun Anti-SARS-CoV-2 ELISA. The other study analyzed blood donor samples with the Wantai Ab ELISA. Data on the sensitivity and specificity of the serological tests were obtained from published research. Time-varying IFR and IHR estimates and their associated delay distributions were estimated within the same model. 1.3. Results By the end of January 2021, we estimated 19% (95%CrI 18%-21%), 13% (95%CrI 12%-15%) and 11% (95%CrI 8%-13%) of the Belgian 18-49, 50-64 and 65-74 year-olds to have been infected with SARS-CoV-2. Infections occurred in two large waves with few infections in between. The first wave mostly affected the younger age group, with a peak weekly incidence of 2.0% (95%CrI 1.7%-2.3%) end of March 2020, while weekly incidences were more comparable during the second wave end of October 2020: 1.6% (95%CrI 1.2%-2%) for 65-74 year-olds and  2.8% (95%CrI 2.3%-3.3%) for 18-49 year-olds.     Both the hospitalization and fatality rates declined over time. Among persons aged 65 to 74 years the hospitalization rate declined from 9.9% (95%CrI 7.2-13.2) to 5.1% (95%CrI 3.2-6.3) and fatality rates from 2.8% (95%CrI 2.0%-3.8%) to 1.3% (95%CrI 0.9%-1.7%). IHR and IFR were considerably lower in the younger age groups.   1.4. Conclusion During 2020, an estimated 16.2% (95%CrI 15.1%-17.3%) of the Belgian adult population was infected with SARS-CoV-2. These infections occurred primarily in two waves, one in March and another one in October 2020, with minimal transmission in between. Severe disease was more prevalent among older age groups and earlier on in 2020.