Antibody Responses in Asymptomatic Close Contacts of COVID-19 Patients and Their Implications

The SARS-CoV-2 belongs to the genus β which has been confirmed to be highly infectious by research. (https://www.who.int/emergencies/ diseases/novel-coronavirus-2019/situation-reports).

The four major structural proteins of coronavirus are the spike surface glycoprotein (S), small envelope protein (E), matrix protein (M), and nucleocapsid protein (N). The spike protein (S) of coronavirus is a type I transmembrane glycoprotein and mediates the entrance to human respiratory epithelial cells by interacting with cell surface receptor angiotensin-converting enzyme 2 (ACE2) , the S protein contains distinct functional domains near the amino (S1) and carboxy (S2) termini, the peripheral S1 portion can independently bind cellular receptors while the integral membrane S2 portion is required to mediate fusion of viral and cellular membranes . The nucleocapsid protein (N) forms complexes with genomic RNA, interacts with the viral membrane protein during virion assembly and plays a critical role in enhancing the efficiency of virus transcription and assembly .

The diagnosis of COVID-19 is dependent mainly on clinical characteristics, CT imaging and a few laboratory tests. Although some symptoms and laboratory parameters have indicative values in confirmed patients, they are not unique to SARSCoV-2 infection. The most used and reliable test for diagnosis of COVID-19 has been the RT-PCR test performed using nasopharyngeal swabs. A variety of RNA gene targets are used by different manufacturers, with most tests targeting 1 or more of the envelope (env), nucleocapsid (N), spike (S), RNA-dependent RNA polymerase (RdRp), and ORF1 genes .

Spread of the COVID-19 global pandemic highlights the urgent need to develop effective treatments or vaccines against SARS-CoV-2 infection. The identification of novel antibodies to neutralize the virus is one of the approaches to fight COVID-19 . In particular, detect an antibody against the SARS-CoV-2 spike protein by building on knowledge about the existing structure of SARS-CoV-2 and learnings from previous SARS antibody generations. Potent neutralizing antibodies often target the receptor interaction site in S1, disabling receptor interactions. The so-called spike protein of 2019-nCoV is used by the virus to dock to human cells. By targeting the protein, to prevent the infection.

COVID-19 patients developed SARS-CoV-2-specific NAbs at the convalescent phase of infection (SARS-CoV-2 NAbs were unable to cross-reactive with SARS-CoV virus). SARS-CoV -2 specific NAbs reached their peak in patients from day 10-15 after the onset of the disease and remained stable thereafter in the patients. Antibodies targeting on different domains of S protein, including S1, RBD, and S2, may all contribute to the neutralization.