Large-Scale Testing Reveals Significant COVID-19 Transmission Risk from Asymptomatic Carriers

Based on the results of the large-scale testing programme run in the Grand Duchy, Research Luxembourg has found that asymptomatic SARS-CoV-2 carriers represent an important vector for transmission.

Large-scale testing for SARS-CoV-2, the virus that causes COVID-19, has been a topic of intense political and societal debate, the rationale for screening asymptomatic individuals having been in particular questioned. Whether asymptomatic carriers play an important role in population-based transmission has remained an essential question.

Luxembourg had been one of the first countries to implement a large-scale testing programme in May 2020, offering all of its residents and cross-border workers the opportunity to regularly and voluntarily get tested for free. The first phase of testing ran from 27 May until 15 September 2020 and covered 49% of the resident and 22% of the cross-border worker populations. This first phase enabled the detection of 26% of all positive cases of the first epidemic wave. According to Research Luxembourg, this provided decision makers with evidence-based data and allowed to break infection chains early whereby the total number of cases would have been 43% higher without the testing over this period, as shown by computer simulations.

In addition, large-scale testing provides a unique test case for assessing the role of asymptomatic carriers. Based on the contact tracing data from phase 1 of the large-scale testing programme, Luxembourg researchers revealed that asymptomatic SARS-CoV-2 carriers are likely similarly infectious compared to symptomatic individuals. Paul Wilmes, Professor of Systems Ecology at the Luxembourg Centre for Systems Biomedicine (LCSB) of the University of Luxembourg, explained: “Indeed, our data show that they infect on average a similar number of people as symptomatic individuals. Asymptomatic carriers, including those that are presymptomatic at the time of the test, thereby represent an important factor in triggering and sustaining infection chains”.

The testing strategy was designed in a three-layered approach by estimating exposure to the disease and physical proximity to categorise activity sectors into high and medium risk. The analysis showed that people who worked in the high-risk sectors had greater odds of being tested positive. Moreover, the data revealed that individuals with a disposable household income of less than €30,000 per year also had the highest risks of being infected.

Ulf Nehrbass, CEO of the Luxembourg Institute of Health (LIH), added: “In addition to its direct effect on infection dynamics, the large-scale testing also enabled the collection of a wealth of pseudonymised data that can now be exploited to learn more about SRAS-CoV-2”.

By clarifying the role of asymptomatic carriers in the transmission of SARS-CoV-2, this analysis provides useful insights for the development of future population-level containment and mitigation strategies.