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 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.

References

  1. Mizumoto K, Kagaya K, Zarebski A, et al. Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Euro Surveill 2020;25.
    doi:
    10.2807/1560-7917.ES.2020.25.10.2000180

  2. Day M. Covid-19: identifying and isolating asymptomatic people helped eliminate virus in Italian village. BMJ 2020;368:m1165.

  3.  Al-Tawfiq JA. Asymptomatic coronavirus infection: MERS-CoV and SARS-CoV-2 (COVID-19). Travel Med Infect Dis 2020;:101608.

  4.  Zhou X, Li Y, Li T, et al. Follow-up of the asymptomatic patients with SARS-CoV-2 infection. Clin Microbiol Infect Published Online First: 28 March 2020.
    doi:
    10.1016/j.cmi.2020.03.024

  5.   Nishiura H, Kobayashi T, Suzuki A, et al. Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19). Int J Infect Dis Published Online First: 13 March 2020. doi:10.1016/j.ijid.2020.03.020

  6.   Kimball A, Hatfield KM, Arons M, et al. Asymptomatic and Presymptomatic SARS-CoV-2 Infections in Residents of a Long-Term Care Skilled Nursing Facility – King County, Washington, March 2020. MMWR Morb Mortal Wkly Rep 2020;69:377–81.

  7.    Quilty BJ, Clifford S, Flasche S, et al. Effectiveness of airport screening at detecting travellers infected with novel coronavirus (2019-nCoV). Eurosurveillance. 2020;25.
    doi:
    10.2807/1560-7917.es.2020.25.5.2000080

  8.  Tian S, Hu N, Lou J, et al. Characteristics of COVID-19 infection in Beijing. J Infect 2020;80:401–6.

  9.    Sun WW, Ling F, Pan JR, et al. [Epidemiological characteristics of 2019 novel coronavirus family clustering in Zhejiang Province]. Zhonghua Yu Fang Yi Xue Za Zhi 2020;54:E027.

  10.     Han Y, Yang H. The transmission and diagnosis of 2019 novel coronavirus infection disease (COVID-19): A Chinese perspective. J Med Virol Published Online First: 6 March 2020. doi:10.1002/jmv.25749

  11.    Qiu H, Wu J, Hong L, et al. Clinical and epidemiological features of 36 children with coronavirus disease 2019 (COVID-19) in Zhejiang, China: an observational cohort study. Lancet Infect Dis Published Online First: 25 March 2020.
    doi:
    10.1016/S1473-3099(20)30198-5

  12.    Anastassopoulou C, Russo L, Tsakris A, et al. Data-based analysis, modelling and forecasting of the COVID-19 outbreak. PLoS One 2020;15:e0230405.

  13.    Day M. Covid-19: four fifths of cases are asymptomatic, China figures indicate. BMJ 2020;369:m1375.

  14.     Hu Z, Song C, Xu C, et al. Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China. Science China Life Sciences. 2020. doi:10.1007/s11427-020-1661-4

  15.     Newman T. COVID-19: Study estimates rate of ‘silent transmission’. Medical News Today. 2020.https://www.medicalnewstoday.com/articles/covid-19-study-estimates-rate-of-silent-transmission (accessed 5 Apr 2020).

  16.     Song H, Xiao J, Qiu J, et al. A considerable proportion of individuals with asymptomatic SARS-CoV-2 infection in Tibetan population.
    doi:10.1101/2020.03.27.20043836

  17.   Q&A: Similarities and differences – COVID-19 and influenza. https://www.who.int/news-room/q-a-detail/q-a-similarities-and-differences-covid-19-and-influenza (accessed 5 Apr 2020).

  18.    Renwick D. Have I already had coronavirus? How would I know and what should I do? the Guardian. 2020.http://www.theguardian.com/us-news/2020/apr/05/have-i-already-had-coronavirus-how-would-i-know (accessed 5 Apr 2020).

  19.    Otmani M. COVID-19: First results of the voluntary screening in Iceland – Nordic Life Science – the leading Nordic life science news service. Nordic Life Science – the leading Nordic life science news service. 2020.https://nordiclifescience.org/covid-19-first-results-of-the-voluntary-screening-in-iceland/ (accessed 5 Apr 2020)

  20.      NPR Choice page. https://www.npr.org/sections/health-shots/2020/03/31/824155179/cdc-director-on-models-for-the-months-to-come-this-virus-is-going-to-be-with-us?t=1586065796492 (accessed 5 Apr 2020).

  21.     CDC. Coronavirus Disease 2019 (COVID-19). Centers for Disease Control and Prevention. 2020.https://www.cdc.gov/coronavirus/2019-ncov/hcp/clinical-guidance-management-patients.html#Asymptomatic (accessed 5 Apr 2020).

  22.    Coronavirus, Castiglione d’Adda è un caso di studio: ‘Il 70% dei donatori di sangue è positivo’. lastampa.it. 2020.https://www.lastampa.it/topnews/primo-piano/2020/04/02/news/coronavirus-castiglione-d-adda-e-un-caso-di-studio-il-70-dei-donatori-di-sangue-e-positivo-1.38666481 (accessed 5 Apr 2020).