Proportion of asymptomatic infection among COVID-19 positive persons and their transmission potential: A systematic review and meta-analysis

PLoS One. 2020 Nov 3;15(11):e0241536. doi: 10.1371/journal.pone.0241536. eCollection 2020.


Background: The study objective was to conduct a systematic review and meta-analysis on the proportion of asymptomatic infection among coronavirus disease 2019 (COVID-19) positive persons and their transmission potential.

Methods: We searched Embase, Medline, bioRxiv, and medRxiv up to 22 June 2020. We included cohorts or cross-sectional studies which systematically tested populations regardless of symptoms for COVID-19, or case series of any size reporting contact investigations of asymptomatic index patients. Two reviewers independently extracted data and assessed quality using pre-specified criteria. Only moderate/high quality studies were included. The main outcomes were proportion of asymptomatic infection among COVID-19 positive persons at testing and through follow-up, and secondary attack rate among close contacts of asymptomatic index patients. A qualitative synthesis was performed. Where appropriate, data were pooled using random effects meta-analysis to estimate proportions and 95% confidence intervals (95% CI).

Results: Of 6,137 identified studies, 71 underwent quality assessment after full text review, and 28 were high/moderate quality and were included. In two general population studies, the proportion of asymptomatic COVID-19 infection at time of testing was 20% and 75%, respectively; among three studies in contacts it was 8.2% to 50%. In meta-analysis, the proportion (95% CI) of asymptomatic COVID-19 infection in obstetric patients was 95% (45% to 100%) of which 59% (49% to 68%) remained asymptomatic through follow-up; among nursing home residents, the proportion was 54% (42% to 65%) of which 28% (13% to 50%) remained asymptomatic through follow-up. Transmission studies were too heterogenous to meta-analyse. Among five transmission studies, 18 of 96 (18.8%) close contacts exposed to asymptomatic index patients were COVID-19 positive.

Conclusions: Despite study heterogeneity, the proportion of asymptomatic infection among COVID-19 positive persons appears high and transmission potential seems substantial. To further our understanding, high quality studies in representative general population samples are required.

Publication types

  • Meta-Analysis
  • Research Support, Non-U.S. Gov't
  • Systematic Review

MeSH terms

  • Asymptomatic Infections / epidemiology*
  • Betacoronavirus / isolation & purification
  • COVID-19
  • Coronavirus Infections / diagnosis*
  • Coronavirus Infections / epidemiology
  • Coronavirus Infections / transmission
  • Coronavirus Infections / virology
  • Databases, Factual
  • Humans
  • Pandemics
  • Pneumonia, Viral / diagnosis*
  • Pneumonia, Viral / epidemiology
  • Pneumonia, Viral / transmission
  • Pneumonia, Viral / virology
  • Prevalence
  • SARS-CoV-2

Grant support

This work was funded by McGill Interdisciplinary Initiative in Infection and Immunity (M[i]4) (ECRF-R1-30), which also supports the salary of MYL and SPA. JRC (Award #258907, Award #287869) is supported by a postdoctoral fellowship from the Fonds de Recherche du Québec—Santé. NW (Award #284837) is funded by a doctoral fellowship from the Fonds de Recherche du Québec—Santé. MB and FF are supported through a Canadian Institutes of Health Research grant (FRD331745). The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.