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

Setting Proportion Comment
Diamond Princess cruise ship, Yokohama, Japan  (n=-634 tested positive). [1] 18%

(95% credible interval  16%-20%).

Most infections occurred before the quarantine start.
Vo’Euganeo, 50 km west of Venice, Italian village  [2] 50% to 75%—were asymptomatic In an open letter to the authorities in the Tuscany region
MERS-CoV [3] Increased from 0% to 29%  over time As the MERS-CoV progressed over time there was more identification of asymptomatic individuals due to increased surveillance and contacts testing.
328 adults in Shanghai [4] 13 (4%)  patients were asymptomatic
Japanese nationals evacuated from Wuhan (n=565) [5] 31% (95% CI: 7.7% to 54%) Based on  temperature screening before disembarkation, interviews on symptoms including fever, cough, and non-specific symptoms
23 Residents of a Long-Term Care Nursing Facility King County, Washington[6] 10 (43%) had symptoms, and 13 (57%) were asymptomatic.

Seven days after testing, 10 of 13 asymptomatics developed symptoms

Symptom-based screening could fail to identify approximately half of nursing home residents with COVID-19.
Airport screening of travellers [7] 17% undetectable by typical screening procedures Based on: A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: Lancet. 2020;0(0):S0140-6736(20)30154-9.  
Hospitalised in Beijing, China (n=262)[8] 13 (5.0%) asymptomatic cases
391 cases including 148 family index cases in Zhejiang Province  [9] 54 (14%)  asymptomatic higher  family  secondary attack rate, the secondary attack rate of spouses is higher than other family members.
Chinese perspective [10] Over the past few days, asymptomatic patients were found in many Chinese cities. Whether asymptomatic people can transmit SARS‐CoV‐2 to others is unclear.

Another uncertainty is whether those who are asymptomatic can cause large‐scale infections.

36 children, Zhejiang, China [11] Asymptomatics, 10 (28%) 7 had acute upper respiratory symptoms (19%)
Data-based analysis, modelling and forecasting of the COVID-19 outbreak  [12] The number of asymptomatic and mild cases with subclinical manifestations that probably did not present to hospitals for treatment may be substantial; these cases, which possibly represent the bulk of the COVID-19 infections, remain unrecognized,
166 new infections in China  [13] four-fifths of cases are asymptomatic, China figures indicate  Numbers quoted  not verifiable
Nanjing, China  (n=24) [14] 5 (21%) developed symptoms (fever, cough, fatigue and etc.) during hospitalization.
450 case reports from 93 Chinese cities. [15] estimate that people who had not yet developed symptoms transmitted around 10% of the cases they studied.
 People’s Hospital of Daofu county. Tibetan population (n=83) [16] Asymptomatic carriers 22% median age of asymptomatic carriers was 31 years and 1/3rd  were students, aged <20 years.
WHO Q&A: Similarities and differences – COVID-19 and influenza [17] suggest that 80% of infections are mild or asymptomatic,
Iceland   [18] 50% of the people who tested positive had no symptoms.” See also: First results of the voluntary screening in Iceland[19]
CDC [20] A significant number of individuals that are infected actually remain asymptomatic. That may be as many as 25%.
Interim Clinical Guidance for Management of Patients with Confirmed Coronavirus Disease [21] Among children in China, illness severity was lower with 94% having asymptomatic, mild or moderate disease,
Northern Italy, 60 volunteer blood donors [22] 40 (67%)  tested positive

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 an Epidemiologist.

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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