A Clinical Comparison of Imported or Locally Acquired COVID-19 Suggests Declining Severity after Viral Passages at the Early Stage of the Pandemic in China

This retrospective, two-center study aimed to provide solid clinical evidence to support the viral attenuation theory after passages of SARS-CoV-2 during the early stages of the pandemic.

Clinical characteristics and outcomes of 150 COVID-19 patients hospitalized in Wenzhou, China between January and April 2020 were compared. These patients had similar demographic characteristics. Infections of 77 patients who visited Wuhan within 14 days before symptom onset were categorized as imported cases and that of the remaining 73 patients who had no recent travel history were categorized as locally transmitted cases.

COVID-19 cases in Wenzhou appeared to be mostly mild at the early stages of the pandemic. There were no differences in clinical manifestations, laboratory testing results, and radiographic presentation between imported and locally transmitted cases in Wenzhou, except that a higher proportion of lymphopenia was found in the imported case group. Assessment of infection severity showed that severe conditions were observed in 10.7% of the patients, with the imported case group having a significantly higher rate (15.6%) than the locally transmitted case group (5.5%, x² = 4.016, p = 0.045).

Although, the clinical manifestations of locally acquired infections were indistinguishable from those imported from Wuhan, they were less likely to develop into severe medical conditions, suggesting the possibility of virulence attenuation after viral passages during the early stages of the pandemic.