Risk of transmission in COVID-19 among close contacts

Risk of transmission in COVID-19 among close contacts. Spencer EA, Heneghan C.


Published on July 6, 2020

Reference Luo L, Liu D, Liao X-I, et al. Modes of contact and risk of transmission in COVID-19 among close contacts. medRxiv 2020.03.24.20042606 2020
Study type
Country China
Setting Community, household, work, transport
Funding Details Guangdong Province 393 Higher Vocational Colleges & Schools Pearl River Scholar Funded 394 Scheme (2019), the Construction of High-level University of Guangdong 395, and the Zhejiang University special 396 scientific research fund for COVID-19 prevention and control 397.
Transmission mode Person to person, Close contact
Exposures Household contact

Bottom Line

2.6% of close contacts of cases contracted COVID-19; almost half were asymptomatic or had  mild infection. The main transmission appeared to take place via household contacts.

Evidence Summary

Among 4,950 close contacts, during quarantine 129 cases (2.6%) were diagnosed, with eight being asymptomatic (6.2%), 49 mild (38%), and five (3.9%) severe to critical cases. 

Among different modes of close contact, household contact was associated with the highest risk of infection with an incidence of 10.2%. 

Increased age of the close contacts was associated with increased risk of infection: 1.8% (0-17 years) to 4.2% (60 or over years)

The severity of source cases was associated with risk of secondary infection: 0.33% for asymptomatic, 3.3% for mild, to 6.2% for severe and critical source cases.

The manifestation of expectoration in source cases was also associated with an increased risk of infection in their close contacts (13.6%). 

Secondary cases were in general clinically milder and were less likely to have common symptoms than those of source cases.

What did they do?

Between 13th January and 6th March 2020, 347 cases of COVID-19 were diagnosed in Guangzhou and their 4,950 close contacts were identified and enrolled in this observational study.

Close contacts included unprotected contacts living in the same household, face-to-face working together, sharing the same classroom, visiting or stay in the same hospital ward, taking the same car or aeroplane, sharing neighbouring seats in the same train or ship as a diagnosed COVID-19 patient, giving direct care to a diagnosed COVID-19 patient.

Data were collected on modes of contact, laboratory testing, clinical characteristics of confirmed cases and the source cases. These data were investigated as risk factors associated with infection among these close contacts, using logistic regression analysis. 

Study reliability

The study was unable to consider the full course of the disease as on the date of data cut-off, some cases were still hospitalised or symptomatic. Also, incident cases were relatively low which limited the power of the analyses. The symptoms assessments  are subject to recall bias.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes Yes Yes Yes Unclear

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Elizabeth Spencer

Dr Elizabeth Spencer; MMedSci, PhD. Epidemiologist, Nuffield Department for Primary Care Health Sciences, University of Oxford.