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