Household secondary attack rate of COVID-19 and associated determinants.

COVID-19: Household secondary attack rate of COVID-19 and associated determinants. Spencer EA, Heneghan C.

Published on June 30, 2020

Reference Jing Q-L, Liu M-J, Yuan et al. Household secondary attack rate of COVID-19 and associated determinants. medRxiv 2020.04.11.20056010 2020
Study type
Country China
Setting Southern China
Funding Details U.S. National Institute of Health , the Science and Technology Plan Project of Guangzhou, the Project for Key Medicine Discipline Construction of Guangzhou Municipality, and the Key research and development program of China.
Transmission mode Person to person, Close contact
Exposures Household

Bottom Line

COVID-19 cases in their presymptomatic period were at least as infectious as during their symptomatic period, and household contacts and elderly people were the most susceptible to infection.

Evidence Summary

195 unrelated clusters with 212 primary cases, 137 nonprimary (hospital) cases and 1,938 uninfected close contacts were traced. 

The household SAR was 13.8% (95%CI: 11.1 to 17.0%) if household contacts were defined as all close relatives and 19.3% (95%CI: 15.5 to 23.9%) if household contacts included only those at the same residential address as the case.

The odds of infection among children (<20 years old) was one quarter (OR 0.26, 95% CI: 0.13 to 0.54) of that among the elderly (≥60 years old). 

There was no gender difference in the risk of infection.  COVID-19 cases were at least as infectious during their incubation period as during their illness. On average, a COVID-19 case infected 0.48 (95% CI: 0.39 to 0.58) close contacts.

What did they do?

Using a comprehensive contact-tracing dataset from Guangzhou for 212 primary cases with 195 unrelated clusters, the population-level effective reproductive number and individual-level secondary attack rate (SAR) in the household setting were estimated. 

The impact of age on transmissibility and the infectivity of COVID-19 cases during their incubation period were investigated.


  • suspected COVID-19 case: a patient meeting ≥1 epidemiological criteria and ≥2 relevant clinical criteria 
  • confirmed case: a suspected case with positive detection of SARSCoV-2 nucleic acid by real-time RT-PCR or viral genes that are highly homologous to SARS-CoV-2 by sequencing using respiratory or blood specimens. 
  • asymptomatic: laboratory confirmation but without clinical signs, mainly found by outbreak investigation and contact tracing.
  • close contact: an individual who had unprotected close contact (within 1 metre) with a confirmed case or an asymptomatic infection within 2 days before their symptom onset or sample collection.

Study reliability

Transmission estimates depend on assumptions of the duration of infectivity at different stages of infection and may introduce bias. The study did not reliably quantify the infectivity of asymptomatic infections given the limited number

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

What else should I consider?

The incubation period being more infectious than the illness period  differs from the  experiences with most respiratory pathogens.

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.