Two proteins have been used as target antigens for serological assays:

• Nucleoprotein (NP): found in virus or virus-infected cells. Due to NP being shielded from antibodies (biological function and location w/in membranes), it’s unlikely that NP antibodies will directly neutralize SARS-CoV-2. In SARS-CoV-1, vaccines containing NP antibodies induced strong antibody response but was non-neutralizing.
• Spike protein: large trimeric glycoprotein that contains the receptor-binding domain, which virus uses to “dock” to cellular receptor angiotensin-converting enzyme 2 (ACE2), and also has the mechanisms that allow fusion of viral and cellular membranes. Because of this, the spike is known as the main, and potentially only target for neutralizing antibodies, hence why spike antibodies are studied in this paper.