Mass Screening for SARS-CoV-2 Uncovers Significant Transmission Risk from Asymptomatic Carriers

Background: To accompany the lifting of COVID-19 lockdown measures, Luxembourg implemented a mass testing programme. The first phase coincided with an early summer epidemic wave. Methods:  High-throughput rRT-PCR was performed using a validated pooling strategy. The sampling infrastructure allowed the testing of the resident and cross-border worker populations. Test strategy was based on social connectivity within different activity sectors. Invitation frequencies were tactically increased in sectors and regions with higher prevalence. The results were analysed alongside contact tracing data. Findings: Sensitivity and specificity of the test protocol were 100%. The tests covered 49% of the resident and 22% of the cross-border worker populations. The programme identified 850 index cases with an additional 249 cases resulting from contact tracing, corresponding to 26% of positive cases of the epidemic wave. Enrichment in positive cases was observed in the services (11·4% increase over the mean prevalence), hospitality (8·6%) and construction (6·6%) sectors alongside regional differences. Strikingly, cases that were asymptomatic on the day of the positive result had a similar secondary attack rate in the household compared to those who were symptomatic. Based on simulations using a tailored agent-based SEIR model, the total number of expected cases would have been 39·1% higher without the mass screening programme. Mandatory participation would have resulted in a further difference of 41·4%. Interpretation: The implementation of strategic and tactical mass testing for SARS-CoV-2 allows the breaking of nascent infection chains and the suppression of epidemic dynamics. Asymptomatic carriers are at least as infectious as symptomatic patients. Containment of future outbreaks will critically depend on early testing in sectors and geographical regions. Higher participation rates must be assured through targeted incentivisation and recurrent invitation. Funding:  This project was funded by the Luxembourg Ministries of Higher Education and Research, and Health. Declaration of Interests: All authors report grants from Luxembourg Ministry of Higher Education and Research, and Ministry of Health during the conduct of the study. Dr. Rodrigues reports working for the Ministry of Higher Education and Research as a public servant, during the conduct of the study. Dr. Snoeck reports that Fast Track Diagnostics provided a few SARS-CoV-2 rRT-PCR kits (RUO) free of charge at the time of evaluation of different commercial assays to be procured by the Luxembourg Government in order to do the mass screening intervention in the country. Ethics Approval Statement: The study was presented to the National Research Ethics Committee of Luxembourg (Comite National d’Ethique de Recherche, CNER) that approved its submission in its current form (ref. 1120-218).