5 mayo, 2021

Testing at scale during the COVID-19 pandemic

Abstract

Assembly and publication of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genome in January 2020 enabled the immediate development of tests to detect the new virus. This began the largest global testing programme in history, in which hundreds of millions of individuals have been tested to date. The unprecedented scale of testing has driven innovation in the strategies, technologies and concepts that govern testing in public health. This Review describes the changing role of testing during the COVID-19 pandemic, including the use of genomic surveillance to track SARS-CoV-2 transmission around the world, the use of contact tracing to contain disease outbreaks and testing for the presence of the virus circulating in the environment. Despite these efforts, widespread community transmission has become entrenched in many countries and has required the testing of populations to identify and isolate infected individuals, many of whom are asymptomatic. The diagnostic and epidemiological principles that underpin such population-scale testing are also considered, as are the high-throughput and point-of-care technologies that make testing feasible on a massive scale.

Introduction

On 7 January 2020, a new coronavirus was detected by metatranscriptomic sequencing of lung fluid from a patient with pneumonia-like symptoms in Wuhan, China^1,2. On 10 January 2020, the assembled reference genome of this new coronavirus, termed severe acute respiratory syndrome (SARS) coronavirus 2 (SARS-CoV-2), was published, and within 2 weeks the first diagnostic tests to detect the virus were issued^3,4. To date, hundreds of millions of individuals have been tested for the presence of SARS-CoV-2 (ref.⁵), leading to widespread public awareness of and debate regarding diagnostic concepts and technologies.

Testing for the presence of SARS-CoV-2 is typically performed for one of two reasons. First, a symptomatic patient might be tested to inform their clinical treatment. Such diagnostic testing is focused on therapeutic care and is typically performed in a well-controlled clinical setting, and the test results are usually interpreted alongside the patient’s history and symptoms. Second, testing might be performed to identify infectious individuals in a population, who are then isolated to prevent the onward infection of others. Such screening is focused on public health outcomes and aims to reduce viral transmission through a population. Individuals who are not symptomatic might be tested, and testing might need to be performed on a massive scale. These two different uses of testing have different requirements and priorities, and a test that is useful in clinical diagnosis can be ill-suited for population-scale screening.

The observation that countries with high testing rates were able to effectively control SARS-CoV-2 transmission during the initial stages of the pandemic⁶ supported the proposal that screening could help to limit viral transmission. Accordingly, widespread testing rapidly gained traction as an intervention that might avoid both the immediate economic costs of lockdown and the societal costs of social distancing measures. Accordingly, many countries implemented population-scale testing to monitor and reduce viral transmission. However, despite the promise of this approach, obtaining reliable test results on such a massive scale is difficult and unprecedented, and has been achieved with differing outcomes and success⁷. Nonetheless, the COVID-19 pandemic has transformed the role of population-scale screening. With ongoing investment and innovation, large-scale testing is likely to become a common feature of public health.

This Review focuses on the use of testing as a tool to screen populations for individuals infected with SARS-CoV-2. We discuss the changing role of such testing in the COVID-19 pandemic: the initial use of genomic epidemiology to monitor the spread of viral strains around the world; the containment of disease outbreaks using contact tracing; and the massive screening programmes that aim to suppress community transmission (Table 1; Fig. 1). We also consider the testing strategies and technologies used to address the challenges of population-scale testing. We finally discuss findings from large-scale testing programmes that have measured the exposure of entire populations to SARS-CoV-2, and how these studies have evaluated and informed our response to the pandemic.