Discussion
We found that the repositive rate of SARS-CoV-2 was over 20% (157/745, 21.1%) among discharged patients affected by COVID-19 at a follow-up visit after at least 6 weeks. They reported positive RT-PCR testing results with median days 8.0 to 14.0 after discharge. Over 4 in 10 children and adolescents were found to be positive again; in contrast, the repositivity rate of SARS-CoV-2 in middle-aged and elderly individuals was 16%. Moreover, patients with more clinically severe disease were less likely to have redetectable positive test results than those with mild severity; asymptomatic patients were most likely to have redetectable positive test results. Manifestation of certain symptoms at first admission, such as fever, was also associated with a lower risk for repositivity. Based on the Chinese guidelines for discharged patients (National_Health_Commission_of_China, 2020), positive retest patients were required to quarantine for a second time. No other positive patients emerged within their families and close contacts.
Several studies, mainly case reports, have been performed to investigate the percentage of retested positivity and the clinical characteristics of discharged patients(Habibzadeh et al., 2020; Lan et al., 2020; Tao et al., 2020). Previous studies reported that the repositivity rate ranged from 6.9% to 69.0% for discharged patients(Habibzadeh et al., 2020; Landi et al., 2020; Tao et al., 2020). However, the studies were limited to a small number of patients with mild or moderate infection. In our study, we evaluated the overall prognosis of patients with COVID-19 after meeting the criteria for discharge in Guangzhou, China. After screening 745 discharged patients, up to September 21, 2020, the repositivity rate was over 20% (157/745), which was higher than that in other cities of China (20/182, 11.0%)(Yuan et al., 2020) and Italy (16.7%)(Landi et al., 2020), and this may be due to the longer follow-up time, more stringent monitoring and higher frequency of RT-PCR testing in Guangzhou. Our study has lasted more than 7 months since the start of the outbreak, which was far longer than other study (most lasted for one or two months)(Lu et al., 2020; Tao et al., 2020; Yuan et al., 2020; Zheng et al., 2020) and to some extent represented the overall prognosis of the disease.
According to the Chinese clinical guidance for COVID-19(National_Health_Commission_of_China, 2020 ), all positive retest patients should test negative for nasopharyngeal and anal swabs for two successive tests before discharge. Then, all discharged patients were continuously quarantined in designated health care facilities with strict interventions on disease transmission. Thus, the identification of another positive SARS-CoV-2 test during the quarantine period likely excludes the possibility that positive retest patients are caused by secondary viral infection. A recent study also experimentally confirmed that the virus was involved in the initial infection instead of a secondary infection(F. Hu et al., 2020). Abnormal CT and lymphopenia are common and correlate with poor clinical outcomes in patients with COVID-19(Cheng et al., 2020). Most positive retest patients at the second admission showed increased lymphocyte cell counts, and CT examination showed abnormal but obvious improvements (Table 2 ), suggesting that positive retest patients have no obvious disease progression but are still asymptomatic carriers of the virus.
Our results showed that the observation of positive retest patients was not random and was mainly observed in young patients without severe clinical symptoms, which was consistent with previous studies(F. Hu et al., 2020; Lu et al., 2020). At present, it has been reported that negative conversion of viral RNA generally takes 2 to 3 weeks or longer(X. Hu et al., 2020; Hung et al., 2020; To et al., 2020; L. Zou et al., 2020), and one study showed that SARS-CoV-2 nucleic acid existed in fecal samples for 47 days after the first symptom onset(Wu et al., 2020). In our study, the days of first hospitalization were shorter in positive retest patients than in negative retest patients (Table 1 ), suggesting that the SARS-CoV-2 virus may not be completely eliminated due to the lighter symptoms and the faster attainment of the discharge standard. In addition, one study reported that recurrently positive RT-PCR testing results in patients with three consecutive negative results were significantly decreased compared with those in patients with two consecutive negative results(Y. Zou et al., 2020), suggesting that a prolonged quarantine phase is necessary.
Among positive retest patients in our study, no families or close contacts of positive retest patients tested positive, which was consistent with current studies(Chandrashekar et al., 2020; Lu et al., 2020). All positive retest patients had observed social distancing measures and worn face masks. Regarding these observations, it is very difficult to affirm whether these patients were truly contagious. RT-PCR testing does not discriminate between an infectious virus and noninfectious RNA(Atkinson & Petersen, 2020; Quick et al., 2017). Therefore, positive testing may not necessarily imply an active infection or ability to transmit infection. However, whether discharged patients have infectivity is an issue of concern around the world at present. One recent study reported that no infectious strain could be obtained by culture, and no full-length viral genomes could be sequenced using samples of positive retest patients(Lu et al., 2020). However, other studies found that active SARS-CoV-2 viral replication was observed(Gousseff et al., 2020) and managed to obtain a nearly full-length viral genome sequence in positive retest patients by detecting intracellular SARS-CoV-2 subgenomic messenger RNA (sgmRNA)(F. Hu et al., 2020), and the presence of SARS-CoV-2 sgmRNA was widely accepted as direct evidence of active viral replication and production(Kim et al., 2020; Wölfel et al., 2020). Furthermore, a recent study found that SARS-CoV-2 viral particles remained in the lungs of patients in the hospital whose nasopharyngeal swab sample testing results were negative at three consecutive times(Yao et al., 2020). Therefore, careful consideration should be given to the potential for patients who are positive retest patients to become chronic virus carriers.
Our study has some limitations. First, as our data were based on the public health response to COVID-19, sample collection did not follow a stringent study design. Therefore, some of the patients, especially in the early stage, had missing fecal samples. Second, nasopharyngeal swab samples cannot differentiate whether the virus comes from the nasopharynx or from secretions from the lower respiratory tract; thus, virus elimination in the lower respiratory tract cannot be confirmed. In contrast, the positive rate of RT‐PCR testing through alveolar lavage fluid may be higher. However, this method is invasive and cannot be widely performed in clinical practice. In our opinion, both qualities of respiratory samples and the variability of technique sensitivity can be attributed to the influencing factors of repositivity. Third, as the discharge patients were usually placed under centralized quarantine and medical observation, the infectivity of the positive retest patients might be underestimated.
In summary, we found that the repositivity rate of discharged patients was relatively high (21.1%), and they tested positive with a median of 8.0 to 14.0 days after discharge. The observation of positive retest patients was not random and was mainly observed in young patients without severe clinical symptoms. We suspected that SARS-CoV-2 may not be completely eliminated in positive retest patients due to the lighter symptoms and the faster attainment of the discharge standard. Meanwhile, as the discharge patients were usually put under centralized quarantine, it is difficult to affirm whether these patients were truly contagious. These findings suggest that a significant proportion of patients could carry viral fragments for a long time, and effective management, such as a prolonged quarantine phase for discharged patients, is necessary.
Acknowledgements. We acknowledge all staffs involved in the prevention and control of COVID-19 at the Guangzhou Centers for Disease Control and Prevention. We thank all patients involved in the study.
Conflicts of interest.All authors report no potential conflicts.
Author Contribution. D.L. contributed to the statistical analyses and drafted the manuscript. C.M., L.L., D.L., Z.H.L., Z.H.W., P.D.Z, X.R.Z., Y.J.Z., W.F.Z. and W.Q.S. revised the final manuscript. Z.B.Z., C.J.X., Z.Q.C., W.T.Z., P.Y. and Q.M.H. collected the epidemiological and clinical data. H.W. and D.L. are responsible for summarizing all epidemiological and clinical data, and contributed to the data cleaning. All authors critically reviewed the manuscript for important intellectual content.
Data Availability Statement. The data that support the findings of this study are available from the corresponding author upon reasonable request.
Financial support. This work was supported by the Guangdong Province Higher Vocational Colleges and Schools Pearl River Scholar Funded Scheme (2019), the Construction of High-level University of Guangdong (G820332010, G618339167 and G618339164), Young Elite Scientists Sponsorship Program by CAST (2019QNRC001), the National Natural Science Foundation of China (82041030), the Zhejiang University special scientific research fund for COVID-19 prevention and control (K920330111).