In December 2019, a novel coronavirus was first reported in Wuhan, China. 1 It was named by the World Health Organization as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and is responsible for coronavirus disease 2019 (COVID-19). Up to 28 February 2020, 79,394 cases have been confirmed according to China’s National Health Commission. Outside China, the virus has spread rapidly to over 36 countries and territories. Cytotoxic lymphocytes such as cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells are necessary for the control of viral infection, and the functional exhaustion of cytotoxic lymphocytes is correlated with disease progression. 2 However, whether the cytotoxic lymphocytes in patients infected with SARS-CoV-2 become functionally exhausted has not been reported. We showed that the total number of NK and CD8+ T cells was decreased markedly in patients with SARS-CoV-2 infection. The function of NK and CD8+ T cells was exhausted with the increased expression of NKG2A in COVID-19 patients. Importantly, in patients convalescing after therapy, the number of NK and CD8+ T cells was restored with reduced expression of NKG2A. These results suggest that the functional exhaustion of cytotoxic lymphocytes is associated with SRAS-CoV-2 infection. Hence, SARS-CoV-2 infection may break down antiviral immunity at an early stage. SARS-CoV-2 has been identified as a genus β-coronavirus, and it shares 79.5% sequence homology with SARS-CoV. 3 In our cohort of 68 COVID-19 patients admitted to The First Affiliated Hospital (Hefei) and Fuyang Hospital (Fuyang), both of which are part of Anhui Medical University in China, there were 55 cases of mild disease (MD) and 13 cases of severe disease (SD). Patients were aged 11–84 years, and the median age of patients was 47.13 years. The percentage of male patients was 52.94%. Consistent with previous studies, many patients had fever (80.88%), cough (73.53%), and sputum (32.36%) upon admission. The prevalence of other symptoms (eg, headache, diarrhea) was relatively low (Supplementary Table 1). The clinical features of patients infected with SARS-CoV-2 was consistent with those reported by Chen and colleagues. 4

Upon admission, the neutrophil count was remarkably higher in SD patients than in MD cases, whereas the lymphocyte count was significantly lower in SD cases than in MD cases. The concentration of total bilirubin, D-dimer, and lactate dehydrogenase in blood was higher in SD patients than that in MD patients. Levels of alanine aminotransferase and aspartate aminotransferase were