Chinese scientists have described the changes of the contents of antibodies to coronavirus in patients with COVID-19 throughout the disease. The results are published on the website of preprints and bioRxiv.
Researchers from Central hospital of Wenzhou in China have observed the dynamics of changes in the content of antibodies to SARS-CoV-2 and viral RNA in 33 patients with COVID-19 during the eight weeks after the first symptoms.
During this period the doctors took 263 samples from the respiratory tract and 135 stool samples to determine viral titer, as well as 171 a sample of blood plasma for the detection of antibodies. Viral load was estimated by polymerase chain reaction with reverse transcription. Data were collected from 27 January to 10 April 2020.
The researchers measured the median time of disappearance of viral RNA in the smear from the throat, sputum, and stool, as well as the dynamics of the content of IgM and IgG antibodies to S – and N-proteins and the receptor-binding domain of the S protein of coronavirus.
Scientists found that in smears from the pharynx and sputum viral load was high in the early stages of infection, but became negative after three weeks after the onset of symptoms. The viral load in feces was initially lower than in smears, but at a slower pace. Many patients with a high titer of viral RNA in the feces lasted more than five weeks.
Thus, the authors believe that the detection of viral RNA in stool samples can be used as an additional analysis, which will help to avoid false-negative results.
The median time of disappearance of viral RNA in smears from the pharynx was 18.5 days, sputum — 22, the chair of -17 days. Three patients showed a relapse of viral RNA in sputum within two weeks after discharge from the hospital, and one patient with persistent viral RNA persisted for 59 days.
Also, researchers found that the level of IgM and IgG antibodies to the receptor-binding domain of the S protein of SARS-CoV-2 was significantly higher in those patients who have rapidly decreased viral titer. The authors conclude that antibodies to this domain effectively fight infection and have the greatest potential for application in clinical practice
It is believed that in plasma during infection appear first IgM and then IgG. According to the survey, 75 per cent of the observed patients IgM and IgG antibodies began to be produced at the same time. However, 10 percent of patients had negative IgM during the study period. Perhaps, the scientists suggest these results are associated with low sensitivity of available tests for IgM.
The content of IgG antibodies in most patients reached a peak about 30 days after the onset of symptoms, and 36 percent of the observed level of IgG to the receptor-binding domain has increased more than four times within one to two weeks after the beginning of antibody production and during the whole time of observation was kept at a high level. In these patients, the viral titer in smears become negative most quickly.
According to the authors of the study, the findings indicate that antibodies can be effectively used for the development of therapeutic methods and drugs.