Waxing and Waning of Serum SARS CoV-2-IgG Level in COVID-19 Exposed Population

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