This study will define the kinetics of IgG responses to both N and S proteins in the subjects who suffered from COVID 19 and then had recovered and those who were previously undiagnosed but were seropositive. These subjects will be followed for four months to evaluate the levels of antibodies in these people.

Since December 2019, cases of unexplained pneumonia have occurred in Wuhan City, Hubei Province, subsequent virus isolation and whole-genome sequencing (accession#: MN908947) confirmed that it is an acute respiratory infection caused by new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) . Coronaviruses are enveloped, non-segmented, single-positive-stranded RNA viruses with round or oval particles and a diameter of 50-200 nm. Coronavirus subfamily is divided into four genera: α, β, γ and δ according to serotype and genomic characteristics. The SARS-CoV-2 belongs to the genus β which has been confirmed to be highly infectious by research.

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 seropositivity rate of both IgM and IgG responses after onset and recovery of disease, and in the context of both N protein and S protein has not been clarified. The kinetics of antibody responses in critical cases or ICU patients has not been reported, and no studies have suggested whether antibody response is associated with disease prognosis

Inclusion Criteria:

• Subjects testing reactive for SARS CoV 2 IgG antibodies

Exclusion Criteria:

• Subjects testing nonreactive for SARS CoV-2 IgG antibodies

subjects who suffered from COVID 19 and then had recovered and those who were previously undiagnosed but were seropositive.