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Characterization of an asymptomatic cohort of SARS-COV-2 infected individuals outside of Wuhan, China

Our take —

This contact tracing study in China showed that 22.3% (63 of 279) of cases testing positive were asymptomatic, and that asymptomatic cases experienced a notable prevalence (46.0%, n=29) of abnormal chest computed tomography scans (CTs). Those with abnormal CTs had longer time to hospital discharge compared to cases with normal CTs. However, this study was relatively small, and all patients received antiviral treatment upon admission, which may have decreased the time until hospital discharge (two negative RT-PCR tests) and impact the generalizability of these findings to other asymptomatic case populations.

Study design

Retrospective Cohort

Study population and setting

This study in Chonqing Three Gorges Center Hospital and Wanzhou District Hospital, identified 279 cases diagnosed with SARS-CoV-2 infection from January to March 2020 via contact tracing and testing, and 63 (22.6%) of those had no symptoms at diagnosis or during hospitalization.

Summary of Main Findings

The study found 63 cases who tested positive for SARS-CoV-2 infection via RT-PCR, but who did not report symptoms. Of these, 29 had abnormal chest CT findings. Those with abnormal chest computed tomography scans (CT) were more likely to be older and male than those with normal chest CT. Those with abnormal chest CT also had longer median time between initial positive test and discharge after two negative tests (13.0 days) compared to those with normal chest CT (10.4 days), but no major differences in laboratory blood testing outcomes.

Study Strengths

The study was able to identify asymptomatic cases via contact tracing and testing and follow their clinical outcomes during their observation periods at these local hospitals. The chest CT data were available for all asymptomatic patients, potentially showing more severe disease despite lack of symptoms. There were also important clinical data available about comorbid conditions and potential exposure history that adds to this study’s strengths.


The study had a small sample size overall, with only 63 asymptomatic cases. Because they are asymptomatic, also, they may be less likely to be identified in a timely manner, and therefore the study may lack precision in estimates of their infectious period. Also, per Chinese standard protocol, all patients received antiviral treatments upon admission. Therefore, these results may not reflect the progression of disease among asymptomatic cases without this intervention, and may not be generalizable to other populations.

Value added

This is an early study of asymptomatic cases with other clinical data, including comorbid conditions, chest radiology, and laboratory serology.