Long-Term Existence of SARS-CoV-2 in COVID-19 Patients: Host Immunity, Viral Virulence, and Transmissibility.

COVID-19 patients can recover with a median SARS-CoV-2 clearance of 20 days post initial symptoms (PIS). However, we observed some COVID-19 patients with existing SARS-CoV-2 for more than 50 days PIS. This study aimed to investigate the cause of viral clearance delay and the infectivity in these patients. Demographic data and clinical characteristics of 22 long-term COVID-19 patients were collected. The median age of the studied cohort was 59.83 ± 12.94 years. All patients were clinically cured after long-term SARS-CoV-2 infection ranging from 53 to 112 days PIS. Peripheral lymphocytes counts were normal. The ratios of interferon gamma (IFN-γ)-secreting cells to total CD4 and CD8 cells were normal as 24.68% ± 9.60% and 66.41% ± 14.87% respectively. However, the number of IFN-γ-secreting NK cells diminished (58.03% ± 11.78%). All patients presented detectable IgG, which positively correlated with mild neutralizing activity (Mean value neutralisation antibodies titers = 157.2, P = 0.05). No SARS-CoV-2 virus was isolated in Vero E6 cells inoculated with nasopharyngeal swab samples from all patients 50 days PIS, and the cytopathic effect was lacking. But one sample was positive for SARS-CoV-2 nucleic acid test in cell supernatants after two passages. Genome sequencing revealed that only three synonymous variants were identified in spike protein coding regions. In conclusion, decreased IFN-γ production by NK cells and low neutralizing antibodies might favor SARS-CoV-2 long-term existence. Further, low viral load and weak viral pathogenicity were observed in COVID-19 patients with long-term SARS-CoV-2 infection.