OBJECTIVE: Traditional Chinese medicine plays a significant role in the treatment of the pandemic of coronavirus disease 2019 (COVID-19). Tanreqing Capsule (TRQC) was used in the treatment of COVID-19 patients in the Shanghai Public Health Clinical Center. This study aimed to investigate the clinical efficacy of TRQC in the treatment of COVID-19. METHODS: A retrospective cohort study was conducted on 82 patients who had laboratory-confirmed mild and moderate COVID-19; patients were treated with TRQC in one designated hospital. The treatment and control groups consisted of 25 and 57 cases, respectively. The treatment group was given TRQC orally three times a day, three pills each time, in addition to conventional Western medicine treatments which were also administered to the control group. The clinical efficacy indicators, such as the negative conversion time of pharyngeal swab nucleic acid, the negative conversion time of fecal nucleic acid, the duration of negative conversion of pharyngeal-fecal nucleic acid, and the improvement in the level of immune indicators such as T-cell subsets (CD3, CD4 and CD45) were monitored. RESULTS: COVID-19 patients in the treatment group, compared to the control group, had a shorter negative conversion time of fecal nucleic acid (4 vs. 9 days, P = 0.047) and a shorter interval of negative conversion of pharyngeal-fecal nucleic acid (0 vs. 2 days, P = 0.042). The level of CD3(+) T cells increased in the treatment group compared to the control group ([317.09 +/- 274.39] vs. [175.02 +/- 239.95] counts/muL, P = 0.030). No statistically significant differences were detected in the median improvement in levels of CD4(+) T cells (173 vs. 107 counts/muL, P = 0.208) and CD45(+) T cells (366 vs. 141 counts/muL, P = 0.117) between the treatment and control groups. CONCLUSION: Significant reductions in the negative conversion time of fecal nucleic acid and the duration of negative conversion of pharyngeal-fecal nucleic acid were identified in the treatment group as compared to the control group, illustrating the potential therapeutic benefits of using TRQC as a complement to conventional medicine in patients with mild and moderate COVID-19. The underlying mechanism may be related to the improved levels of the immune indicator CD3(+) T cells.