Reducing contacts to stop SARS-CoV-2 transmission during the second pandemic wave in Brussels, Belgium

Background: Reducing contacts is a cornerstone of containing SARS-CoV-2. We evaluated the effect of physical distancing measures and of school reopening on contacts and consequently on SARS-CoV-2 transmission in Brussels, a hotspot during the second European wave. Methods: Using SARS-CoV-2 case reports and contact tracing data during August-November 2020, we estimated changes in the age-specific number of reported contacts. We associated these trends with changes in the instantaneous reproduction number Rt and in age-specific transmission-events during distinct intervention periods in the Brussels region. Furthermore, we analysed trends in age-specific case numbers, pre- and post-school opening. Findings: When schools reopened and physical distancing measures relaxed, the weekly mean number of reported contacts surged from 2.01 (95%CI 1.73-2.29) to 3.04 (95%CI 2.93-3.15), increasing across all ages. The fraction of cases aged 10-19 years started increasing before school reopening, with no further increase following school reopening (risk ratio 1.23, 95%CI 0.79-1.94). During the subsequent month, 8.9% (67/755) of infections identified were from teenagers to other ages, while 17.0% (131/755) from other ages to teenagers. Rt peaked mid-September at 1.48 (95%CI 1.35-1.63). Reintroduction of physical distancing measures reduced reported contacts to 1.85 (95%CI 1.78-1.91), resulting in Rt dropping below 1 within 3 weeks. Interpretation: The second pandemic wave in Brussels was the result of increased contacts across all ages following school reopening. Stringent physical distancing measures, including closure of bars and limiting close contacts while schools remain open, reduced social mixing, in turn controlling SARS-CoV-2 transmission. Funding: European Commission H2020. GGC Brussel.