The pandemic of coronavirus diseases 2019 (COVID-19) imposes a heavy burden on medical resources [1]. Whether there is correlation between viral load and disease severity has not been clarified. In the study, we retrospectively collected the virological data, as well as demographic, epidemiological clinical information of 92 patients with confirmed COVID-19 in a single hospital in Zhejiang Province, China. We compared the baseline viral loads between severe patients and those mild to moderate at admission and also between those developing severe disease during hospitalization and those not.

We studied 92 patients with confirmed COVID-19 who were admitted from January 19, 2020, to March 19, 2020, in the First Affiliated Hospital of Zhejiang University. The sputum specimens were collected from the lower respiratory tract of each patient at admission and the levels of viral nuclei acid were determined by a real-time PCR (RT-PCR) approach and indicated by the cycle threshold (Ct) values of RT-PCR assays [2]. Other demographic, epidemiological and clinical information were collected and inputted into a pre-designated electronic data collection form. All patients followed up to March 15, 2020. All the statistical analyses were performed using GraphPad Prism 5 (GraphPad Software Inc.; San Diego, CA, USA) and SPSS 20.0 (SPSS Inc.; Chicago, IL, USA).

Of the 92 patients, 30 were severe on admission. Of the other 62 mild-moderate cases at admission, 11 cases became severe during hospitalization. The demographic, epidemiological and clinical information was shown in Table 1. All patients were tested for SARS-CoV-2 nucleic acid on sputum specimens from the lower respiratory tract at admission. As shown in Fig. 1a, severe patients had significantly lower Ct values than mild-moderate cases at admission (25 vs. 28, p = 0.017), suggesting a higher viral load in the lower respiratory tract. Furthermore, a higher viral load was observed in sputum specimens from patients who became severe during the hospitalization than those did not (24 vs. 29, p = 0.008). As shown in Fig. 1b, the Ct values of RT-PCR assays negatively correlated with the probability of progression to severe type in all the patients representing mild-to-moderate at admission.

Table 1 Demographic, comorbidities, epidemiological characteristics, and clinical and laboratory findings of patients with confirmed COVID-19 at admission
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Fig. 1
figure 1

a Comparison of baseline sputum viral load between severe and mild-to-moderate patients at admission or between those becoming severe and those did not during the hospitalization. b Relationship between the estimated probability of disease progression during the hospitalization and baseline sputum viral load. Viral load is indicated by the Ct value of RT-PCR assay. The asterisk indicates a P value < 0.05

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We found that the viral load of the sputum specimen in the lower respiratory tract tested at baseline is closely related to the severity of COVID-19. More importantly, patients with a higher baseline viral load are more likely to become severe. This finding apparently justifies the concept that early antiviral treatment, if effective, would reduce the risk of progression and thereby the mortality, which has been demonstrated in influenza [3]. In our study, sputum specimens were used, instead of nasopharyngeal and oropharyngeal swabs because it has been shown that samples from lower respiratory tract generally contain a higher level of viral load than nasopharyngeal and oropharyngeal swabs [4] and acquiring swabs is uncomfortable for patients.

In summary, we found a positive association between sputum viral load and disease severity as well as risk of progression.