The role of asymptomatic SARS-CoV-2 infections: systematic review

The role of asymptomatic SARS-CoV-2 infections: systematic review. Spencer EA,  Heneghan C.

Published on July 23, 2020

Reference Diana C Buitrago-Garcia, Dianne Egli-Gany, Michel J Counotte, Stefanie Hossmann, Hira Imeri, Aziz Mert Ipekci, Georgia Salanti, Nicola Low. The role of asymptomatic SARS-CoV-2 infections: rapid living systematic review and meta-analysis. medRxiv 2020.04.25.20079103 2020
Study type
Country Global
Setting Systematic review
Funding Details Swiss National Science Foundation; European Union Horizon 2020 research and innovation programme, project EpiPose
Transmission mode Presymptomatic, Asymptomatic

Bottom Line

Evidence from early in the pandemic suggests that most SARS-CoV-2 infections are not asymptomatic throughout the course of infection, and that transmission by presymptomatic accounts for around 40-60% of transmission and asymptomatic cases accounts for around 15% of transmission.

Evidence Summary

The overall estimate of the proportion of people who become infected with SARS-CoV-2 and remain asymptomatic throughout infection was 15% (95% CI 10 to 22%) with a prediction interval of 3 to 55% in 28 studies that addressed this review question. 

There was some evidence that bias in the selection of participants influences the outcome. 

The proportion of people that is pre-symptomatic could not be summarised, owing to heterogeneity. 

In modelling studies, 40 to 60% of all SARS-CoV-2 infections are the result of transmission from pre-symptomatic individuals, with a smaller contribution from asymptomatic individuals. 

What did they do?

Rapid living systematic review and meta-analysis set out to answer the following questions

  1. amongst people who become infected with SARS-CoV-2, what proportion does not experience symptoms at all during their infection? 
  2. Amongst people with SARS-CoV-2 infection who are asymptomatic when diagnosed, what proportion will develop symptoms later? 
  3. What proportion of SARS-CoV-2 transmission is accounted for by people who are either asymptomatic throughout infection, or pre-symptomatic? 

The review searched PubMed, Embase, bioRxiv and medRxiv using a living evidence database of SARS-CoV-2 literature, searched on 25 March 2020 and updated on 20 April 2020. 

Studies of people with SARS-CoV-2 diagnosed by reverse transcriptase PCR that documented follow-up and symptom status at the beginning and end of follow-up or modelling studies. 

One reviewer extracted data and a second verified the extraction, with disagreement resolved by discussion or a third reviewer. Risk of bias was assessed with an adapted checklist for case series and a questionnaire to assess relevance and credibility of modelling studies.

Study reliability

High quality systematic review based on studies of variable quality. Some included studies were at high risk of bias due to selecting participants based on symptom status. 

This study was not able to consider the impact of false negative tests on the results.

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

What else should I consider?

Included studies:

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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.