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Contact Settings and Risk for Transmission in 3410 Close Contacts of Patients With COVID-19 in Guangzhou, China: A Prospective Cohort Study

Our take —

The study objective was to describe the secondary attack rate of COVID-19 in Guangzhou, China based on exposure setting. The study found that household transmission carried the highest risk, with 10.3% secondary infection rate, while public transport had the lowest with 0.1% secondary infection rate. Severity of symptoms was also related to secondary attack rates. The study may not be generalizable to other settings because of prevention measures in China, including that people are quickly moved to hospitals for monitoring after testing positive. Still, these data show that the exposure setting is important to assessing the risk of secondary infection, and that those with severe symptoms are more likely to transmit the virus compared to asymptomatic cases.

Study design

Prospective Cohort

Study population and setting

The study objective was to identify the amount of secondary cases of COVID-19 in Guangzhou, China identified through contact tracing of index cases. Index cases were identified by the Guangzhou CDC from December 28, 2019 to March 5, 2020. From the 391 index cases (244 in Guangzhou and 147 from other areas) identified, 3410 close contacts were also identified. The study collected data about exposure setting, defined as household, public transportation, health care settings, entertainment venues or workplaces, and multiple settings considered as more than one of the other four categories. Contacts were quarantined for 14 days from last contact and followed up prospectively through April 6, 2020, with symptom monitoring conducted morning and evening, with RT-PCR testing for SARS-CoV-2.

Summary of Main Findings

Of the 3410 close contacts, 127 (4.0%) were infected as either asymptomatic or symptomatic cases, and ended quarantine after a median of 2 days (IQR: 1 to 5 days) largely due to being transferred to hospitals soon after being identified as a case. Household exposure had the highest risk for secondary infection, with 10.3% (95% CI: 8.5 to 12.2%), while the lowest risk exposure settings were those on public transportation, with 0.1% infection risk (95% CI: 0.00 – 0.4%). Asymptomatic cases had a secondary attack rate of 0.3% (95% CI: 0.0 to 1.0%), while there was a dose-response relationship with increasing attack rates for those with mild vs. moderate vs. severe cases respectively. Those with severe case contacts had the highest attack rate with 6.2% (95% CI: 3.2 to 9.1%).

Study Strengths

The study identified a large number of close contacts and tested each of them regularly, which likely identified asymptomatic cases to better estimate the attack rate. The study also collected a number of exposure variables to understand different contact settings, which is important for policymakers making recommendations. The study also identified contacts and cases through a robust surveillance system, which identified people through surveillance testing, healthcare facility testing, and close contact testing in order to identify many index cases during the study period.


The study identified a small number of secondary cases, which means that the models had wide confidence intervals and may have imprecise point estimates. Additionally, because a number of infection control efforts had been implemented, including moving positive cases to hospital settings for monitoring, these results are unlikely to be generalizable to other settings (e.g., the median duration of quarantine was 2 days because people were moved to hospitals so quickly).

Value added

This study estimates the secondary attack rate among a range of exposure settings, using robust surveillance and contact tracing methods.

This review was posted on: 3 September 2020