Study population and setting
The study collected data on the age and sex distributions, the proportion of locally transmitted cases, and the proportion of cases associated with exposure to known cases, among laboratory-confirmed cases in China outside of Hubei province, from December 2019 to February 17, 2020. Individual-level information was used to estimate the distributions of time from symptom onset to hospital admission, time from exposure to illness onset (incubation period), and time between symptom onset of primary and secondary cases(serial interval). These quantities were estimated separately for the time periods before and after January 27, 2020. For nine locations with sufficient data, the study estimated the effective reproduction number over time.
Summary of Main Findings
Between the first and second time period, the proportion of cases with exposure to COVID-19 cases or patients with acute respiratory infections increased from 26% to 66%, and the proportion of cases with exposure to Hubei decreased from 73% to 35%. The mean time from symptom onset to quarantine (through hospital admission) improved from 4.4 days to 2.6 days (p < 0.0001). Across the two time periods, the mean incubation period was 5.2 days (95th percentile at 10.5 days), and the mean serial interval was 5.1 days (with 95% of serial intervals between 1.3 and 11.6 days). Because a large proportion of serial intervals are smaller than the mean incubation period, a substantial amount of transmission could occur before symptom onset, potentially making contact tracing, isolation of infectious individuals, and airport screening difficult. In all nine locations, the effective reproduction number had decreased below 1 by February 8, 2020, indicating that interventions had effectively controlled local epidemics.
The availability of individual-level data by location, and data on transmission pairs, enabled estimation of the distributions of times to key events, and changes in the effective reproduction number by location. Stratifying these distributions by time period showed the shift in local epidemics from imported cases to local transmission, but also revealed improved epidemic control over time. The individual data collected for this study were made public, enabling further analyses.
The study did not explicitly account for expanding case definitions over time, and so the estimated effective reproduction numbers should be treated as upper bounds. The results of the study are also sensitive to changes in reporting rates over time.
This study was one of the first to characterize transmission in China outside Hubei. It revealed that key quantities for epidemic control such as the serial interval and mean time to isolation were shorter than those estimated for Hubei for previous studies, suggesting better control of the epidemic outside Hubei.
This review was posted on: 21 July 2020