Maximum sensitivity is ensured through detection of antibodies against both proteins making the assay superior compared to other commercially available serology tests where false negative results may occur. Measurement of both RBD and NP IgG antibodies also provides an opportunity to discriminate vaccinated versus naturally infected individuals. Spike is one of the major vaccine targets against SARS-CoV-2 with antibodies against Spike RBD correlating well with protection while Nucleocapsid is an immunodominant protein expressed with high immunogenicity during the early stages of SARS-CoV-2 infection.
Highly sensitive and specific serological assays are crucial for surveillance, to help uncover the burden of disease and to get accurate estimates of morbidity and mortality associated with SARS-CoV-2. Serological assays are also required to evaluate immunological response at an individual/population level and during vaccine trials.
Crucially, simultaneous measurement of antibodies to RBD and NP provides a tool to monitor breakthrough SARS-CoV-2 infection in vaccinated individuals. This testing approach has wider societal implications in the context of measuring ongoing vaccine effectiveness against evolving SARS-CoV-2 variants such as the Delta variant.
The S protein consisting of S1 and S2 subunits is located on the surface of the SARS-CoV-2 virus making it highly immunogenic. The S protein plays an important role in viral attachment, fusion and entry to the host cell, as such it serves as a target for the development of antibodies, entry inhibitors and vaccines. The Receptor Binding Domain (RBD) located on the S1 subunit is the key target for neutralising antibodies.