COVID-19: What proportion are asymptomatic?

April 6, 2020

Carl Heneghan, Jon Brassey, Tom Jefferson

Readers of the blog will be aware that we have little trust in the current reported COVID data. Everywhere we look we cannot get a handle on the essential facts or at times we get 2 completely different answers to the same question. The military historian Sir Basil Liddle Hart would have called this “the fog of a pandemic” or perhaps the “fog of information overload”.

So, we thought we would try to answer an important question: what is the proportion of people with SARS-COV-2 who are asymptomatic?

To answer this we searched LitCovid (a subset of Pubmed), medRxiv, Trip, Scholar and Google. We retrieved 21 reports for analysis.

What did we learn (see the table for the analysis)

That between 5% and 80% of people testing positive for SARS-CoV-2 may be asymptomatic

That symptom-based screening will miss cases, perhaps a lot of them

That some asymptomatic cases will become symptomatic over the next week (sometimes known as “pre-symptomatics”)

That children and young adults can be asymptomatic

We also learnt that there is not a single reliable study to determine the number of asymptotics. It is likely we will only learn the true extent once population based antibody testing is undertaken.

Table of studies assessing asymptomatic cases

Sir Basil’s “fog” is continuing to cover the topic and prevent us from seeing what lies on the other side of the hill.

Tom Jefferson is a senior associate tutor and honorary research fellow, Centre for Evidence-Based Medicine, University of Oxford.

Disclosure statement is here

Carl Heneghan is Professor of Evidence-Based Medicine, Director of the Centre for Evidence-Based Medicine and Director of Studies for the Evidence-Based Health Care Programme. (Full bio and disclosure statement here)

Jon Brassey is the Director of Trip Database Ltd, Lead for Knowledge Mobilisation at Public Health Wales (NHS) and an Associate Editor at the BMJ Evidence-Based Medicine

Disclaimer: the article has not been peer-reviewed; it should not replace individual clinical judgement and the sources cited should be checked. The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the NHS, the NIHR, or the Department of Health and Social Care. The views are not a substitute for professional medical advice.

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