To our knowledge, no previous studies have focused on determining whether the virulence and case fatality rate of the severe acute respiratory coronavirus 2 (SARS-CoV-2) decreases as the virus continues to spread. Hence, our aim was to retrospectively explore the differences in the risk of severe or critical COVID-19 among imported, secondary and tertiary cases in Zhejiang, China.
We categorized COVID-19 cases reported by hospitals in Zhejiang as first-, second- and third-generation cases. Univariate and multivariate logistic regression analyses were performed to compare disease severity and case generation.
Of 1187 COVID-19 cases, 227 (19.1%, 95% CI: 16.9–21.4) manifested severe or critical illness. The adjusted risk difference for severe or critical illness was lower for second- (odds ratio (OR) = 0.84, 95% confidence interval (CI): 0.52–1.36) and third-generation (OR = 0.55, 95% CI: 0.37–0.83) cases than for first-generation cases. Compared with hospitalized patients, cases identified at centralized isolation locations (OR = 0.62, 95% CI: 0.40–0.97) and those identified through active search or gateway screening (OR = 0.28, 95% CI: 0.08–1.04) were at a lower risk of severe or critical illness.
Second- and third-generation cases of COVID-19 have a lower risk of developing severe or critical illness than first-generation cases.