A simple, rapid assay to evaluate neutralizing antibodies that target the SARS-CoV-2 spike protein receptor binding domain (RBD) is described in a paper published in Nature Biotechnology. The assay, which does not require the use of the live virus, was validated in two groups of patients who had COVID-19 from Singapore and Nanjing, China. It is much faster than conventional tests for neutralizing antibodies—taking only 1–2 hours to complete—and does not need to be performed in special biosafety laboratories.
A rapid test to detect neutralizing antibodies to SARS-CoV-2 is urgently needed to facilitate monitoring of infection rates, herd immunity and protective immunity, as well as vaccine efficacy during clinical trials and after large-scale vaccination.
The current gold standard assay for detecting neutralizing antibodies requires the handling of live SARS-CoV-2 in a biosafety level 3 laboratory containment facility and is time-consuming, taking 2–4 days to complete. The pseudovirus-based virus neutralization test—another type of test that detects neutralizing antibodies—can be done in a biosafety level 2 laboratory, but still requires the use of live viruses and cells.
Lin-Fa Wang, Danielle Anderson and colleagues designed a surrogate virus neutralization test that does not require the use of any live virus or cells. This test takes 1–2 hours to complete and can be conducted in a biosafety level 2 laboratory, which is the same as an ELISA test. The authors used the purified RBD from the viral spike protein and the host cell receptor ACE2 to mimic the virus–host interaction in an enzyme-linked immunosorbent assay plate. This interaction can be blocked by specific neutralizing antibodies in patient or animal sera in a similar manner to the conventional virus neutralization and pseudovirus-based virus neutralization tests. The assay can also differentiate neutralizing antibodies from antibodies that bind RBD, but do not neutralize the virus.
The authors validated their test with two separate groups of patients recovering from COVID-19, in Singapore (175 patients who had COVID-19 and 200 healthy controls) and Nanjing, China (50 patients who had COVID-19 and 200 healthy controls). The authors tested different panels and confirmed that their test can distinguish between antibody responses to COVID-19 and to other human coronavirus infections. They found that their assay achieves 99.93% specificity and 95–100% sensitivity. When the authors studied the specificity of their test for SARS-CoV-2 versus the earlier SARS virus using serum collected from patients who had recovered from the 2003 epidemic, they noted that SARS-neutralizing antibodies are still detectible 17 years later.
The authors note that, although the surrogate virus neutralization assay may never be able to completely replace the conventional virus neutralization assay, it performs well and in some cases may be easier for the broader community to use for many aspects of COVID-19 investigation.
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