Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
Review
. 2020 Dec 31;5(4):223-234.
doi: 10.3138/jammi-2020-0030. eCollection 2020 Dec.

Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: Systematic review and meta-analysis

Oyungerel Byambasuren  1 , Magnolia Cardona  1 , Katy Bell  2 , Justin Clark  1 , Mary-Louise McLaws  3 , Paul Glasziou  1
Affiliations
Review

Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: Systematic review and meta-analysis

Oyungerel Byambasuren et al. J Assoc Med Microbiol Infect Dis Can. .

Abstract
in English, French

Background: Knowing the prevalence of true asymptomatic coronavirus disease 2019 (COVID-19) cases is critical for designing mitigation measures against the pandemic. We aimed to synthesize all available research on asymptomatic cases and transmission rates.

Methods: We searched PubMed, Embase, Cochrane COVID-19 trials, and Europe PMC for primary studies on asymptomatic prevalence in which (1) the sample frame includes at-risk populations and (2) follow-up was sufficient to identify pre-symptomatic cases. Meta-analysis used fixed-effects and random-effects models. We assessed risk of bias by combination of questions adapted from risk of bias tools for prevalence and diagnostic accuracy studies.

Results: We screened 2,454 articles and included 13 low risk-of-bias studies from seven countries that tested 21,708 at-risk people, of which 663 were positive and 111 asymptomatic. Diagnosis in all studies was confirmed using a real-time reverse transcriptase-polymerase chain reaction test. The asymptomatic proportion ranged from 4% to 41%. Meta-analysis (fixed effects) found that the proportion of asymptomatic cases was 17% (95% CI 14% to 20%) overall and higher in aged care (20%; 95% CI 14% to 27%) than in non-aged care (16%; 95% CI 13% to 20%). The relative risk (RR) of asymptomatic transmission was 42% lower than that for symptomatic transmission (combined RR 0.58; 95% CI 0.34 to 0.99, p = 0.047).

Conclusions: Our one-in-six estimate of the prevalence of asymptomatic COVID-19 cases and asymptomatic transmission rates is lower than those of many highly publicized studies but still sufficient to warrant policy attention. Further robust epidemiological evidence is urgently needed, including in subpopulations such as children, to better understand how asymptomatic cases contribute to the pandemic.

Historique: Il est essentiel de connaître la prévalence des véritables cas asymptomatiques de maladie à coronavirus 2019 (COVID-19) pour concevoir des mesures d’atténuation de la pandémie. Les chercheurs ont voulu synthétiser toutes les recherches disponibles sur les cas asymptomatiques et les taux de transmission.

Méthodologie: Les chercheurs ont fouillé les bases de données PubMed, Embase, Cochrane pour trouver les études sur la COVID-19, et Europe PMC pour colliger les études primaires sur la prévalence des cas asymptomatiques dans lesquelles 1) le cadre d’échantillonnage incluait une population à risque et 2) le suivi était suffisant pour dépister les cas présymptomatiques. La méta-analyse a fait appel à des modèles d’effets fixes et d’effets aléatoires. Nous avons évalué le risque de biais par une combinaison de questions adaptées d'outils sur les risques de biais des études de prévalence et de précision diagnostique.

Résultats: Les chercheurs ont extrait 2 454 articles, dont 13 études à faible risque de biais de sept pays dans lesquelles 21 708 personnes à risque ont subi le test de dépistage, soit 663 cas positifs et 111 cas asymptomatiques. Dans toutes les études, le diagnostic a été confirmé au moyen du test d’amplification en chaîne par polymérase après transcriptase inverse en temps réel. La proportion de cas asymptomatiques se situait entre 4 % et 41 %. La méta-analyse (à effets fixes) a établi que la proportion de cas asymptomatiques s’élevait à 17 % (IC à 95 %, 14 % à 20 %) dans l’ensemble, mais qu’elles étaient plus élevées dans les soins aux aînés (20 %; IC à 95 %, 14 % à 27 %) qu’auprès du reste de la population (16 %; IC à 95 %, 13 % à 20 %). Le risque relatif [RR] de transmission de cas asymptomatiques était plus faible de 42 % que celui de cas symptomatiques (RR combiné de 0,58; IC à 95 %, 0,34 à 0.99, p = 0,047).

Conclusions: L’évaluation de la prévalence d’un sixième de cas asymptomatiques de COVID-19 et de taux de transmission de cas asymptomatiques est inférieure à celle de nombreuses études hautement publicisées, mais suffit tout de même pour justifier l’intérêt de la santé publique. D’autres données épidémiologiques solides s’imposent de toute urgence, y compris dans des sous-populations comme les enfants, pour mieux comprendre l’effet des cas asymptomatiques sur la pandémie.

Keywords: emerging or re-emerging diseases; epidemiology; evidence-based medicine; public health policy.

PubMed Disclaimer

Conflict of interest statement

Mary-Louise McLaws is a member of the World Health Organization (WHO) Health Emergencies Program Experts Advisory Panel for Infection Prevention and Control (IPC) Preparedness, Readiness and Response to COVID-19 and WHO IPC Guidance Development Group for COVID-19.

Figures

Figure 1:
Figure 1:
Depiction of ideal study flow and criteria used for study inclusion: (1) sample frame of at-risk people and (2) adequate follow-up on symptoms SARS-Cov-2 = Severe acute respiratory syndrome coronavirus 2; PCR = Polymerase chain reaction
Figure 2:
Figure 2:
Screening and selection of articles
Figure 3:
Figure 3:
Pooled estimates of proportion of asymptomatic carriers by subpopulations N = Positive cases; n = Asymptomatic cases
Table 3:
Table 3:
Risk of bias in 13 included studies*
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

See all "Cited by" articles

References

    1. Yuen K-S, Ye ZW, Fung S-Y, Chan C-P, Jin D-Y. SARS-CoV-2 and COVID-19: the most important research questions. Cell Bioscience. 2020;10(1):40. 10.1186/s13578-020-00404-4. Medline: - DOI - PMC - PubMed
    1. Bai Y, Yao L, Wei T, et al. Presumed asymptomatic carrier transmission of COVID-19. JAMA. 2020;323(14):1406–7. 10.1001/jama.2020.2565. Medline: - DOI - PMC - PubMed
    1. Mizumoto K, Kagaya K, Zarebski A, Chowell G. 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(10). 10.2807/1560-7917.ES.2020.25.10.2000180. Medline: - DOI - PMC - PubMed
    1. Heneghan C, Brassey J, Jefferson T. COVID-19: what proportion are asymptomatic? Centre for Evidence-Based Medicine, Oxford, UK. 2020. https://www.cebm.net/covid-19/covid-19-what-proportion-are-asymptomatic/ (April 5, 2020).
    1. Clark J, Glasziou P, Del Mar C, Bannach-Brown A, Stehlik P, Scott AM. A full systematic review was completed in 2 weeks using automation tools: a case study. J Clin Epidemiol. 2020;121:81–90. 10.1016/j.jclinepi.2020.01.008. Medline: - DOI - PubMed

LinkOut - more resources