Study population and setting
This study utilized contact-tracing surveillance data collected from January 28 to March 15, 2020 in Guangzhou, China. These data included a total of 359 confirmed COVID-19 cases, and 369 close contacts of those cases to estimate the secondary attack rate based on the proportion of COVID-19 incidence among close contacts of pre-symptomatic cases. The eventual symptomatic status of the pre-symptomatic cases was determined based on the person’s clinical course assessed by a physician as of March 30, 2020.
Summary of Main Findings
Overall, among the 369 close contacts, 12 secondary cases were identified through diagnostic testing, resulting in a secondary attack rate of 3.3% (95% CI 1.9%–5.6%), with estimates of 16.1% among household members, 1.5% among contacts with friends or family, 1.1% among social contact with strangers, and 0% among workplace contacts. Secondary attack rates varied by eventual symptomatic status of index cases, including 0.8% among contacts of asymptomatic cases, 3.5% to 5.7% among contact of pre-symptomatic index cases who eventually became symptomatic.
This study incorporates both pre-symptomatic and asymptomatic index cases in the analyses of onward transmission to contacts.
This study is limited to close contacts who were able to be reached, and therefore may be subject to selection bias. The study may have missed asymptomatic index cases and contacts, who therefore may not be represented. Close contacts who were exposed to more than two confirmed COVID-19 cases were excluded from this study. Because there were a limited number of secondary cases (n=12), confidence intervals were wide when comparing the relative risk of secondary infection based on index case symptoms, suggesting that a larger sample size is necessary to ascertain patterns between symptom severity and secondary transmission.
This study considers the eventual clinical presentation of index cases in onward transmission to secondary cases during the pre-symptomatic period.
This review was posted on: 17 June 2020