Risk of Secondary COVID-19 Transmission Low in Most Settings


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The risk of secondary transmission is less than 4% overall among close contacts of people diagnosed with COVID-19 and varies by settings and disease severity, new research shows.

Lei Luo, PhD, with the Guangzhou Center for Disease Control and Prevention, China, and colleagues used contract tracing to test 3410 close contacts of 391 COVID-19 index cases between January and March in Guangzhou.

Researchers identified cases through surveillance testing, screening symptomatic patients who presented to a healthcare facility, or tracing and screening people in close contact with those diagnosed with COVID-19.

Among the close contacts, 127 (3.7%) became infected. Of those 127, eight (6.3%) were asymptomatic. Of the 119 symptomatic cases, 20 (16.8%) were mild, 87 (73.1%) were moderate, and 12 (10.1%) were severe or critical.

The findings were published online August 13 in the Annals of Internal Medicine.

To learn how risk varied by contact locations, they stratified contacts according to where they interacted with the index patient, including household, public transportation, health care settings, entertainment venues or workplaces, and multiple settings.

Risks for secondary infection were highest for household contacts, at 10.3% (95% CI, 8.5% – 12.2%), followed by those exposed in health care settings, at 1.0% (95% CI, 0.3% – 1.8%; odds ratio [OR] relative to household exposure, 0.09), and on public transportation, at 0.1% (95% CI, 0.0% – 0.4%; OR, 0.01).

The authors note that the 10.3% household transmission rate was consistent with previously reported secondary infection rates among household members of 11.2% in other cities in China and 10.5% in the United States.

Disease Severity Correlates With Risk

Researchers found that the risk for secondary transmission increased with the severity of index cases.

“This supports the view of the World Health Organization that asymptomatic cases were not the major drivers of the overall epidemic dynamics,” the authors write.

Table. Secondary Transmission by Severity of Index Cases

Severity level Transmission (%)* 95% CI, %
Asymptomatic 0.3 0.0 – 1.0
Mild 3.3 1.8 – 4.8
Moderate 5.6 4.4 – 6.8
Severe/critical 6.2 3.2 – 9.1

*P for trend < 0.001.


When index case patients had expectoration, they were linked with nearly five times the risk for secondary transmission (13.6% vs 3.0% for index cases without expectoration; OR, 4.81; 95% CI, 3.35 – 6.93).

China is able to trace close contacts of people diagnosed with COVID-19 through a cellphone database developed based on the movements of users. The system is able to measure proximity between people when cases are confirmed.

Researchers acknowledged a study limitation was that patients with COVID-19 were asked to remember when symptoms first occurred, which may introduce recall bias. Additionally, symptoms and severity of index cases were not assessed at the time of exposure to contacts. Funding was provided by the Guangdong Province Higher Vocational Colleges and Schools Pearl River Scholar Funded Scheme, the National Natural Science Foundation of China, the Construction of High-Level University of Guangdong, and the Zhejiang University Special Scientific Research Fund for COVID-19 Prevention and Control. The authors have reported no relevant financial relationships.

Ann Intern Med. Published online August 13, 2020. Full text

Marcia Frellick is a freelance journalist based in Chicago. She has previously written for the Chicago Tribune, Science News, and Nurse.com and was an editor at the Chicago Sun-Times, the Cincinnati Enquirer, and the St. Cloud (Minnesota) Times. Follow her on Twitter at @mfrellick

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