A systematic review of SARS-CoV-2 transmission

A systematic review of SARS-CoV-2 transmission. Jefferson T, Heneghan C.


Published on June 22, 2020

Reference Koh WC, Naing L, Rosledzana MA, Alikhan MF, Chaw L, Griffith M, et al. What do we know about SARS-CoV-2 transmission? A systematic review and meta-analysis of the secondary attack rate, serial interval, and asymptomatic infection. medRxiv. 2020 2020
Study type
Country Various
Setting Household and non-household
Funding Details Unfunded
Transmission mode Person to person, Close Contact
Exposures Enclosed settings

Bottom Line

The mean household SAR was 15% (95% CI: 12% to 19%) – ranging from 6.6% in Taiwan to more than 30% in four Chinese cities. Secondary attack rate from symptomatic index cases contact was greater than asymptomatics

Evidence Summary

The secondary attack rate (SAR) is defined as the probability that an exposed susceptible person develops disease caused by an infected person.The mean household SAR is 15% (95% CI: 12% to 19%) – ranging from 6.6% in Taiwan to more than 30% in four Chinese cities including Wuhan

The mean non-household rate was 4.0% (95% CI: 2.8% to 5.2%) with wide variation across the settings: 73% in a chalet, 53% in a choir, 18% dining with a case and 15% in a church event.

Secondary attack rates from symptomatic index cases contacts were 2.5 times greater than those of asymptomatic.

The serial interval was defined as the time between disease symptom onset of a case and that of its infection. Asymptomatic cases were defined as those with laboratory confirmation but without clinical signs and symptoms at diagnosis (including pre-symptomatic).

Asymptomatic cases:

The mean asymptomatic proportion at diagnosis was 26% (95% CI: 19%, 33%), decreasing with age (42%) in children.

What did they do?

The authors performed a search for studies in PubMed and pre-print articles in medRxiv and bioRxiv between 1st  of  January and 15th of  May 2020 with slightly different inclusion criteria according to the fixed variable of interest: secondary attack rate (20 studies) across different settings; clinical onset serial interval (18 studies), and the proportion of asymptomatic infection (66 studies). They only included studies reporting original data, thus excluding models. Although they pooled data they did not carry out a methodological quality assessment.

Study reliability

There is considerable heterogeneity amongst the studies that undermine the pooled estimates. 

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes Yes Yes Yes No

What else should I consider?

This evidence from this review needs updating and increasing. Although a wide variety of settings were included in the analysis, ultimately each setting cell had several small studies which introduced heterogeneity perhaps because of the way they had been carried out in the midst of a pandemic.  We were unable to identify a protocol for the review. In the absence of a predefined protocol, studies are subject to significant uncertainties due to changes in the analysis that may not be clear.

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.

Tom Jefferson

Tom Jefferson, epidemiologist.