A novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged in Wuhan, China, to cause an epidemic of severe pneumonia, now called coronavirus disease (COVID-19), which spread to other parts of China and to the rest of the world . The disease is now a pandemic with, as at 22 April 2020, more than 2.4 million confirmed cases reported to the World Health Organization (WHO) from multiple continents, leading to more than 162,000 deaths . While there are estimates of disease severity and infection attack rates, the true severity of disease remains a major knowledge gap because mild or asymptomatic infections are difficult to estimate . The invisible ‘iceberg’ of mild infections needs to be estimated to fully assess disease severity, as was done during the 2009 influenza A(H1N1)pdm09 pandemic using population-based sero-epidemiology . These studies allowed us to accurately estimate the true age-specific hospitalisation rates, intensive care admission rates and deaths . Such information is crucial in order to assess development of herd immunity and to calibrate our response to this pandemic.
In order to carry out age-stratified population-based sero-epidemiology, it is important to validate serological methods that can be used in such large-scale studies. Ideally, we need highly sensitive high-throughput assays for rapid screening large numbers of sera and highly specific assays that can be used to confirm those sera identified to be positive in the screening tests. These assays also need to be evaluated in different specimen types (e.g. serum, plasma) to maximise the available options for study design. We developed an ELISA assay based on the recombinant receptor-binding domain (RBD) of the SARS-CoV-2 spike protein for use as a screening assay and micro-neutralisation (MN) and plaque reduction neutralisation tests (PRNT) using live virus in biosafety level 3 containment as confirmatory tests. We evaluate the sensitivity and specificity of each of these tests in a cohort of patients with virologically confirmed COVID-19 disease and in an age-stratified set of control sera collected before the emergence of COVID-19 to serve as a negative control population.