LETTERS https://doi.org/10.1038/s41591-020-0965-6 Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections Quan-Xin Long 1,8, Xiao-Jun Tang2,8, Qiu-Lin Shi2,8, Qin Li3,8, Hai-Jun Deng 1,8, Jun Yuan1, Jie-Li Hu1, Wei Xu2, Yong Zhang 2, Fa-Jin Lv4, Kun Su3, Fan Zhang5, Jiang Gong5, Bo Wu6, Xia-Mao Liu7, Jin-Jing Li7, Jing-Fu Qiu 2 ✉ , Juan Chen 1 ✉ and Ai-Long Huang 1 ✉ The clinical features and immune responses of asymptomatic understanding of the clinical features and immune responses of individuals infected with severe acute respiratory syndrome asymptomatic individuals with SARS-CoV-2 infection is limited. coronavirus 2 (SARS-CoV-2) have not been well described. We Here we describe the epidemiological and clinical characteristics, studied 37 asymptomatic individuals in the Wanzhou District virus levels and immune responses in 37 asymptomatic individuals. who were diagnosed with RT–PCR-confirmed SARS-CoV-2 infections but without any relevant clinical symptoms in the Results preceding 14 d and during hospitalization. Asymptomatic Demographic characteristics. On February 6, 2020, the National individuals were admitted to the government-designated Health Commission of China updated the COVID-19 Prevention Wanzhou People’s Hospital for centralized isolation in accor- and Control Plan (4th edition) for the management of close con- dance with policy1. The median duration of viral shedding in tacts, emphasizing identification and quarantine of asymptomatic the asymptomatic group was 19 d (interquartile range (IQR), individuals1. To identify asymptomatic individuals, the Wanzhou 15–26 d). The asymptomatic group had a significantly lon- District Centers for Disease Control and Prevention (CDC) then ger duration of viral shedding than the symptomatic group conducted extensive RT–PCR screening for 2,088 close con- (log-rank P = 0.028). The virus-specific IgG levels in the tacts under quarantine. Individuals with positive RT–PCR results asymptomatic group (median S/CO, 3.4; IQR, 1.6–10.7) were then were screened by point prevalence surveys carried out by significantly lower (P = 0.005) relative to the symptomatic the local CDC and symptoms assessments reported by clinicians. group (median S/CO, 20.5; IQR, 5.8–38.2) in the acute phase. Of these, 60 individuals claimed no symptoms in the preced- Of asymptomatic individuals, 93.3% (28/30) and 81.1% ing 14 d, according to local CDC records, and were transferred (30/37) had reduction in IgG and neutralizing antibody lev- to a government-designated hospital for centralized isolation. On els, respectively, during the early convalescent phase, as com- admission, 17 individuals were excluded for mild or atypical symp- pared to 96.8% (30/31) and 62.2% (23/37) of symptomatic toms based on symptoms assessments reported by clinicians; six patients. Forty percent of asymptomatic individuals became individuals who developed symptoms 4–17 d after admission were seronegative and 12.9% of the symptomatic group became also excluded. Finally, 37 asymptomatic cases, defined as individu- negative for IgG in the early convalescent phase. In addition, als with a positive nucleic acid test result but without any relevant asymptomatic individuals exhibited lower levels of 18 pro- clinical symptoms in the preceding 14 d and during hospitalization, and anti-inflammatory cytokines. These data suggest that were included in this study. A total of 178 patients with confirmed asymptomatic individuals had a weaker immune response to SARS-CoV-2 infections were identified in the Wanzhou District SARS-CoV-2 infection. The reduction in IgG and neutralizing before April 10, 2020, as tracked by CDC surveillance systems. In antibody levels in the early convalescent phase might have this study, the proportion of patients with asymptomatic infections implications for immunity strategy and serological surveys. was 20.8% (37/178). As of May 24, 2020, the coronavirus disease 2019 (COVID-19) For antibody detection and cytokine measurements, 37 pandemic, caused by SARS-CoV-2, has affected more than 5 million sex-, age-frequency- and comorbidity-matched mild symptom- people around the world. Most patients with SARS-CoV-2 infec- atic patients were selected for comparison with the asymptom- tions have reportedly had mild to severe respiratory illness with atic individuals (Supplementary Table 1). Thirty-seven sex- and symptoms such as fever, cough and shortness of breath, which might age-frequency-matched control individuals from the Wanzhou appear 2–14 d after exposure. However, there are other patients who District with negative RT–PCR results for SARS-CoV-2 were also are diagnosed by a positive RT–PCR test but are either asymptom- selected for cytokines comparison. atic or minimally symptomatic2–6. Increasing evidence has shown Of the 37 asymptomatic individuals, the median age was 41 years that asymptomatic individuals can spread the virus efficiently, (range, 8–75 years) and 22 were female. Twenty-eight individuals and the emergence of these silent spreaders of SARS-CoV-2 has had a confirmed history of contact with an RT–PCR-confirmed caused difficulties in the control of the epidemic2,5. However, our patient with COVID-19, and nine were Wuhan residents or had a 1Key Laboratory of Molecular Biology on Infectious Diseases, Ministry of Education, Chongqing Medical University, Chongqing, China. 2School of Public Health and Management, Chongqing Medical University, Chongqing, China. 3Chongqing Center for Disease Control and Prevention, Chongqing, China. 4Department of Radiology, The First Affiliated Hospital of Chongqing Medical University, Chongqing, China. 5Wanzhou People’s Hospital, Chongqing, China. 6Wanzhou District Center for Disease Control and Prevention, Chongqing, China. 7The Second Affiliated Hospital of Chongqing Medical University, Chongqing, China. 8These authors contributed equally: Quan-Xin Long, Xiao-Jun Tang, Qiu-Lin Shi, Qin Li, Hai-Jun Deng. ✉e-mail: [email protected]; [email protected]; [email protected] 1200 NatURE MEDICINE | VOL 26 | AUGust 2020 | 1200–1204 | www.nature.com/naturemedicine NATURE MEDICINE LETTERS ab Fig. 1 | Chest CT scans from two asymptomatic patients. a, CT scan of a 45-year-old female showing focal ground-glass opacities in the lower lobe of the left lung (arrow). b, CT scan of a 50-year-old female showing ground-glass opacities and stripes coexisting in the lower lobe of the right lung (arrows). travel history to Wuhan before the onset of infection (Supplementary a P = 0.336 P = 0.126 Table 2). 40 Radiologic and laboratory findings. A complete blood count, 35 blood biochemistry, coagulation function, liver and renal func- tion and infection biomarkers were measured upon admission 30 Asymptomatic Symptomatic (Supplementary Table 2) to monitor the potential disease progres- 25 sion, according to the COVID-19 Treatment Guidelines (5th edi- Cycle threshold 7 tion) from the National Health Commission of China . Of the 37 20 asymptomatic individuals, three had lymphopenia and one had thrombocytopenia. Six individuals had elevated levels of alanine 15 aminotransferase, and 11 had increased C-reactive protein levels. ORF1ab ORF1ab N N (n = 37) (n = 37) (n = 37) (n = 37) Upon admission, chest computed tomography (CT) scans showed focal ground-glass opacities in 11 asymptomatic individu- b 1.0 als (11/37, 29.7%) and stripe shadows and/or diffuse consolidation Asymptomatic cases (n = 37) 0.9 in ten individuals (10/37, 27.0%), whereas 16 individuals (16/37, Symptomatic cases (n = 37) 43.2%) had no abnormalities (Fig. 1). Five individuals developed 0.8 focal ground-glass opacities or stripe shadows on chest CT within 0.7 5 d of hospital admission. There were no pleural effusions, air 0.6 bronchogram signs or enlarged lymph nodes, which were typi- cal changes seen in critically symptomatic patients8–10. Abnormal 0.5 radiological findings confined to one lung were identified in 66.7% 0.4 (14/21) of the asymptomatic individuals, whereas 33.3% (7/21) had e rate of viral RNA (%) 0.3 abnormalities in both lungs. ositiv 0.2 P 0.1 Hazard ratio = 1.69 (1.06–2.70) Virological outcomes. We compared the RT–PCR cycle thresh- Log rank P = 0.028 old (Ct) values of the first positive nasopharyngeal swabs for all 0.0 37 asymptomatic individuals and 37 symptomatic patients. The 0510 15 20 25 30 35 40 45 initial Ct values for 37 asymptomatic individuals and 37 symptom- Duration of viral shedding (d) atic patients appeared similar (ORF1b 32.8 (IQR, 30.9–35.8) versus 31.7 (IQR, 30.3–35.1), P = 0.336; N 32.6 (IQR, 29.5–34.6) versus Fig. 2 | Virological characteristics in asymptomatic and symptomatic 33.5 (IQR, 31.3–37.2), P = 0.126) (Fig. 2a). The median duration of cases. a, The Ct values of ORF1b and N obtained with RT–PCR that were viral shedding, defined as the interval from the first to last posi- detected in nasopharyngeal swabs from asymptomatic (n = 37) and tive nasopharyngeal swab, in the asymptomatic individuals was 19 symptomatic (n = 37) groups. The box plots show the medians (middle d (IQR, 15–26 d). The shortest observed duration of viral shedding line) and the first and third quartiles (boxes), whereas the whiskers show was 6 d, whereas the longest was 45 d. The median duration of viral 1.5× the IQR above and below the box. Unpaired, two-sided Mann–Whitney shedding was 14 d (IQR, 9–22 d) in patients with mild symptoms. U test P values are depicted in the plots, and the significant P value cutoff The asymptomatic group had a significantly longer duration of was set at 0.05. b, The Kaplan–Meier method was used to estimate the viral shedding than the symptomatic group (log-rank P = 0.028) positive rate of viral RNA, and the two-sided log-rank test was applied to (Fig.