Serological assays for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), March 2020

BACKGROUND: The ongoing coronavirus disease (COVID-19) pandemic has major impacts on health systems, the economy and society. Assessing infection attack rates in the population is critical for estimating disease severity and herd immunity which is needed to calibrate public health interventions. We have previously shown that it is possible to achieve this in real time to impact public health decision making. AIM: Our objective was to develop and evaluate serological assays applicable in large-scale sero-epidemiological studies. METHODS: We developed an ELISA to detect IgG and IgM antibodies to the receptor-binding domain (RBD) of the spike protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We evaluated its sensitivity and specificity in combination with confirmatory microneutralisation (MN) and 90% plaque reduction neutralisation tests (PRNT(90)) in 51 sera from 24 patients with virologically confirmed COVID-19 and in age-stratified sera from 200 healthy controls. RESULTS: IgG and IgM RBD ELISA, MN and PRNT(90) were reliably positive after 29 days from illness onset with no detectable cross-reactivity in age-stratified controls. We found that PRNT(90) tests were more sensitive in detecting antibody than MN tests carried out with the conventional 100 tissue culture infectious dose challenge. Heparinised plasma appeared to reduce the infectivity of the virus challenge dose and may confound interpretation of neutralisation test. CONCLUSION: Using IgG ELISA based on the RBD of the spike protein to screen sera for SARS-CoV-2 antibody, followed by confirmation using PRNT(90), is a valid approach for large-scale sero-epidemiology studies.