Many European countries are reeling under a second wave of the coronavirus disease 2019 (COVID-19) pandemic. In many cases – as in the UK, Italy and Belgium – this has forced countries to move back into nation-wide lockdowns. While the test-trace-isolate approach still remains the most effective way to control the epidemic rebound after the lockdown ends, there is a lot of debate among experts on optimal ways to use testing.
Paris, France - November 13 2020: Covid-19 Rapid testing medical tent in front of a pharmacy. Image Credit: UlyssePixel / Shutterstock
To date, testing is mostly targeted at symptomatic individuals and contacts of positive cases. The increasing availability of new diagnostic tests enables mass testing to be considered as a strategy. Mass testing helps test a large part of the population through a single campaign and identify and isolate several infected individuals at the same time.
The development of rapid tests to detect SARS-CoV-2 has facilitated the implementation of mass testing, as these tests provide results within 30 minutes compared to standard PCR tests that take 1-2 days to provide results. Although the sensitivity of antigenic tests in detecting SARS-CoV-2 are not on par with the PCR tests, the sensitivity threshold of the most sensitive antigenic tests is sufficient to identify a huge proportion of infectious individuals with very high viral shedding. However, even with these rapid antigenic tests, mass testing implementation is very challenging, and its impact is yet to be determined.
Using a mathematical model to assess the impact of mass testing during a rebound of SARS-CoV-2
A team of researchers from various institutions in France recently used a mathematical model to assess the impact of mass testing in controlling the SARS-CoV-2 epidemic. They evaluated the potential impact of mass testing campaigns in an epidemic rebound scenario at the end of the ongoing lockdown in Metropolitan France. Their study has been published on the pre-print server, medRxiv*.
The team used a compartmental SEEIR model to describe the spread of SARS-CoV-2 in Metropolitan France. They found that after infection, most individuals move within 4 compartments. 1. Compartment E1, where they have been exposed but are not infectious yet, with an average duration of 4 days; 2. Compartment E2, where they are infectious and can transmit disease but are not showing symptoms, with an average duration of 1 day; 3. Compartment I, where they are infectious and may be symptomatic, with an average duration of 3 days; and 4. Compartment R, where they have recovered. This description indicates a generation time of around 7 days, which is consistent with existing data on SARS-CoV-2 transmission chains and viral shedding.
Mass testing is not a ‘silver bullet’ that can replace other control measures
The findings of their analysis suggest that in order to positively impact epidemic dynamics, mass testing should be targeted on a large part of the population, be repeated, and should rely on a rapid test with good sensitivity so as to identify all infectious individuals. The researchers also say that their findings emphasize that the strategy works only if a large proportion of individuals testing positive are effectively isolated.
Our analysis suggests that, to positively impact epidemic dynamics, mass testing needs to target a large proportion of the population, be repeated, and rely on a test with good sensitivity to identify infectious individual.”
Using a simple mathematical model, this study highlighted the potential and limitations of mass testing as a strategy to control the SARS-CoV-2 pandemic. A key limitation of mass testing is that, at the beginning of the campaign, about half of the infected individuals are still in the latent phase E1 where they do not shed enough virus to test positive. These undetected individuals will become infectious post the campaign, thus fueling the epidemic once again.
Based on the findings, the researchers concluded that the conditions needed to control a rapidly growing epidemic using mass testing seem impossible to achieve. Hence, mass testing should not be deemed as a silver bullet that can replace other control measures. According to the authors, even with a set optimistic assumptions, mass testing is most relevant when epidemic growth is limited and only as a part of a combination of interventions.
Mass testing should therefore not be seen as a silver bullet that will ensure other control measures can be removed.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.