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. 2021 May:4:100056.
doi: 10.1016/j.lanepe.2021.100056. Epub 2021 Feb 27.

SARS-CoV-2 transmission risk from asymptomatic carriers: Results from a mass screening programme in Luxembourg

Paul Wilmes  1   2 , Jacques Zimmer  3 , Jasmin Schulz  3   4 , Frank Glod  4 , Lisa Veiber  5 , Laurent Mombaerts  1   6 , Bruno Rodrigues  7 , Atte Aalto  1 , Jessica Pastore  4 , Chantal J Snoeck  3 , Markus Ollert  3 , Guy Fagherazzi  4 , Joël Mossong  8 , Jorge Goncalves  1 , Alexander Skupin  1 , Ulf Nehrbass  4
Affiliations

SARS-CoV-2 transmission risk from asymptomatic carriers: Results from a mass screening programme in Luxembourg

Paul Wilmes et al. Lancet Reg Health Eur. 2021 May.

Abstract

Background: To accompany the lifting of COVID-19 lockdown measures, Luxembourg implemented a mass screening (MS) programme. The first phase coincided with an early summer epidemic wave in 2020.

Methods: rRT-PCR-based screening for SARS-CoV-2 was performed by pooling of samples. The infrastructure allowed the testing of the entire resident and cross-border worker populations. The strategy relied 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: The voluntary programme covered 49% of the resident and 22% of the cross-border worker populations. It identified 850 index cases with an additional 249 cases from contact tracing. Over-representation was observed in the services, hospitality and construction sectors alongside regional differences. Asymptomatic cases had a significant but lower secondary attack rate when compared to symptomatic individuals. Based on simulations using an agent-based SEIR model, the total number of expected cases would have been 42·9% (90% CI [-0·3, 96·7]) higher without MS. Mandatory participation would have resulted in a further difference of 39·7% [19·6, 59·2].

Interpretation: Strategic and tactical MS allows the suppression of epidemic dynamics. Asymptomatic carriers represent a significant risk for transmission. Containment of future outbreaks will depend on early testing in sectors and 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.

Keywords: Asymptomatic; COVID-19; Contact tracing; Mass screening; SARS-CoV-2; rRT-PCR.

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Conflict of interest statement

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.

Figures

Fig. 1:
Fig. 1
Epidemic curve for COVID-19 in Luxembourg.
Fig. 2:
Fig. 2
Strategic and tactical mass testing. The frequencies of invitations resulted in (A) high-frequency testing for high-risk sectors and (B) medium-frequency testing for medium-risk sectors. (C) The prevalence in the population was monitored at low testing frequency and targeted invitations were sent in the case of regional flare-ups.
Fig. 3:
Fig. 3
Flowchart detailing numbers of contacts resulting from the 850 identified index cases.
Fig. 4:
Fig. 4
Testing in the construction sector since its reopening.
Fig. 5:
Fig. 5
Cumulative numbers of cases per 100 000 population, by canton.
Fig. 6:
Fig. 6
Impact analysis by agent-based modelling (A) Daily detected cases from the simulated actual scenario together with 7-day moving average of true data. (B, C) The second wave in terms of active cases (B) and total cases (C) in the actual scenario, a scenario without mass screening, and a scenario with full participation. (D) The effect of trigger date and the numbers of tests in regional targeting in relation to the reduction in total cases.
See this image and copyright information in PMC

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