SARS-CoV-2 serology testing: Progress and challenges

Abstract

The coronavirus disease 2019 (COVID-19) pandemic has caused the most devasting social and economic impact of this century. The current pandemic will end only after a safe, effective vaccine becomes available and protective herd immunity has been achieved through vaccination. The key parameter to gauge protective immunity is neutralizing antibody levels. Thus, reliable serology testing is essential to diagnose whether an individual has been previously infected, as a large proportion of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections is asymptomatic. For both naturally infected and vaccinated individuals, it is critical to monitor their neutralizing antibody titers over time. This is because, when neutralizing antibody levels wane below a threshold which remains to be determined, they become vulnerable to reinfection. Due to the importance of serology testing, academia and industry have developed different platforms for serological diagnosis, many of which have achieved the Food and Drug Administration (FDA) Emergency Use Authorizations (EUA). Here we summarize the status of COVID-19 serology testing, discuss challenges, and provide future directions for improvement.

Keywords: Binding antibodies; COVID-19; Neutralizing antibodies; SARS-CoV-2; Serology testing.

Fig. 1

Assays for the detection of neutralizing antibodies (nAb). A. Plaque-reduction neutralization test (PRNT) is the current gold standard for detection of nAb. When nAb are absent, spike proteins of SARS-CoV-2 interact with ACE2 receptors (ACE2) to allow viral entry, replication, and subsequent plaque formation. Presence of nAb inhibits viral entry and the formation of plaques. B. A surrogate neutralization assay using a blocking ELISA: horseradish peroxidase (HRP) conjugated to the receptor-binding domain (RBD) of the SARS-CoV2 spike protein is pre-incubated on ACE2R-coated ELISA plate. If nAb are present, HRP-conjugated RBD is blocked from binding, resulting in an attenuated signal when 3,3′,5,5′-tetramethylbenzidine (TMB) substrate is provided. C. Reporter virus neutralization assays are based on the expression level of reporter signal in infected cells. Reporter signal will occur when there is viral entry into cells. When nAb are present, inhibition of entry causes decreased reporter signal from cells. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)