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. 2021 Dec 14;4(12):e2137257. doi: 10.1001/jamanetworkopen.2021.37257

Global Percentage of Asymptomatic SARS-CoV-2 Infections Among the Tested Population and Individuals With Confirmed COVID-19 Diagnosis

A Systematic Review and Meta-analysis

Qiuyue Ma 1, Jue Liu 1, Qiao Liu 1, Liangyu Kang 1, Runqing Liu 2, Wenzhan Jing 1, Yu Wu 1, Min Liu 1,✉
PMCID: PMC8672238  PMID: 34905008

Key Points

Question

What is the percentage of asymptomatic individuals with positive test results for SARS-CoV-2 among tested individuals and those with confirmed COVID-19 diagnosis?

Findings

In this systematic review and meta-analysis of 95 unique studies with 29 776 306 individuals undergoing testing, the pooled percentage of asymptomatic infections was 0.25% among the tested population and 40.50% among the population with confirmed COVID-19.

Meaning

The high percentage of asymptomatic infections from this study highlights the potential transmission risk of asymptomatic infections in communities.

This systematic review and meta-analysis evaluated the percentage of asymptomatic COVID-19 infections among individuals undergoing testing and those with confirmed infections.

Abstract

Importance

Asymptomatic infections are potential sources of transmission for COVID-19.

Objective

To evaluate the percentage of asymptomatic infections among individuals undergoing testing (tested population) and those with confirmed COVID-19 (confirmed population).

Data Sources

PubMed, EMBASE, and ScienceDirect were searched on February 4, 2021.

Study Selection

Cross-sectional studies, cohort studies, case series studies, and case series on transmission reporting the number of asymptomatic infections among the tested and confirmed COVID-19 populations that were published in Chinese or English were included.

Data Extraction and Synthesis

This meta-analysis was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. Random-effects models were used to estimate the pooled percentage and its 95% CI. Three researchers performed the data extraction independently.

Main Outcomes and Measures

The percentage of asymptomatic infections among the tested and confirmed populations.

Results

Ninety-five unique eligible studies were included, covering 29 776 306 individuals undergoing testing. The pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.23%-0.27%), which was higher in nursing home residents or staff (4.52% [95% CI, 4.15%-4.89%]), air or cruise travelers (2.02% [95% CI, 1.66%-2.38%]), and pregnant women (2.34% [95% CI, 1.89%-2.78%]). The pooled percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 33.50%-47.50%), which was higher in pregnant women (54.11% [95% CI, 39.16%-69.05%]), air or cruise travelers (52.91% [95% CI, 36.08%-69.73%]), and nursing home residents or staff (47.53% [95% CI, 36.36%-58.70%]).

Conclusions and Relevance

In this meta-analysis of the percentage of asymptomatic SARS-CoV-2 infections among populations tested for and with confirmed COVID-19, the pooled percentage of asymptomatic infections was 0.25% among the tested population and 40.50% among the confirmed population. The high percentage of asymptomatic infections highlights the potential transmission risk of asymptomatic infections in communities.

Introduction

COVID-19, the disease caused by SARS-CoV-2, was first reported in December 2019.1 Globally, as of January 28, 2021, there have been 100 455 529 confirmed cases, including 2 166 440 deaths.2 The disease course of COVID-19 ranges from asymptomatic to mild respiratory infections to pneumonia and even to acute respiratory distress syndrome.3 Patients with no symptoms at screening point were defined as having asymptomatic infections, which included infected people who have not yet developed symptoms but go on to develop symptoms later (presymptomatic infections), and those who are infected but never develop any symptoms (true asymptomatic or covert infections).4,5 Owing to the absence of symptoms, these patients would not seek medical care and could not be detected by temperature screening. Presymptomatic transmission will also make temperature screening less effective.6 Only extensive testing and close contact tracing could lead to identification of more asymptomatic infections.7

Unlike SARS, which had little known transmission from asymptomatic patients, evidence showed that asymptomatic patients were a potential source of transmission of COVID-19.3,6 A previous study8 showed that the upper respiratory viral loads in asymptomatic patients were comparable to those in symptomatic patients. Meanwhile, the highest viral load in throat swabs at the time of symptom onset indicated that infectiousness peaked on or before symptom onset.9 Moreover, studies showed that asymptomatic infections might have contributed to transmission among households, nursing facilities, and clusters.10,11,12,13 As the pandemic has been contained in many countries and regions, travel restrictions have been lifted and public places have reopened. Asymptomatic infections should be considered a source of COVID-19 infections that play an important role in the spread of the virus within community as public life gradually returns to normal. The management of asymptomatic carriers was essential for preventing cluster outbreaks and transmission within a community.

However, comprehensive evaluation of the percentage of asymptomatic infections among the tested population and the population with confirmed COVID-19 (confirmed population) is limited. Current results from different studies3,5,7,8,10,11 varied considerably owing to different study design and study population. Thus, we conducted a meta-analysis to better understand the global percentage of asymptomatic infections among the tested and confirmed COVID-19 populations. Our results could be useful for strategies to reduce transmission by asymptomatic infections.

Methods

Search Strategy

We conducted the meta-analysis following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. This review was not registered. Three researchers (Q.L., L.K., and R.L.) searched the published studies on February 4, 2021, through PubMed, EMBASE, and ScienceDirect without language restriction. The search terms used included COVID-19, coronavirus, SARS-CoV-2, asymptomatic transmission, asymptomatic infection, asymptomatic proportion, asymptomatic case, asymptomatic cases, asymptomatic contact, asymptomatic ratio, asymptomatic people, asymptomatic patients, and asymptomatic patient. The detailed search strategies are shown in eMethods 1 in the Supplement. Three researchers (Q.L., L.K., and R.L.) reviewed the titles, abstracts, and full texts of articles independently and identified additional studies from the reference lists. Disagreements were resolved by 2 other reviewers (W.J. and Y.W.).

Selection Criteria

Asymptomatic individuals with positive test results for SARS-CoV-2 (asymptomatic infections) were defined as those who did not present any symptoms at the time of SARS-CoV-2 testing or diagnosis.14 Individuals with a confirmed COVID-19 diagnosis were defined as those who had a throat swab or other specimen with positive results for SARS-CoV-2 using a real-time reverse-transcription polymerase chain reaction assay. Inclusion criteria consisted of (1) studies reporting the number of asymptomatic infections, tested population, and confirmed population and (2) cross-sectional studies, cohort studies, case series studies, and case series on transmission. Exclusion criteria consisted of (1) reviews, systematic reviews, and meta-analysis; (2) duplicate publications; (3) preprints; (4) multiple studies reporting on overlapping participants (the study with more information was included); (5) articles with ambiguous definition of asymptomatic infections; and (6) articles not written in English or Chinese.

Data Extraction and Quality Assessment

Three researchers (Q.L., L.K., and R.L.) performed the data extraction independently. Data were extracted for the first author, date of publication, study location, number of tested individuals, number of individuals with confirmed COVID-19, and number of asymptomatic infections. The ratio of male to female individuals (MFR) and mean age of study participants were gathered if available. The quality of studies included in the meta-analysis was assessed using the Joanna Briggs Institute Prevalence Critical Appraisal Tool15 for cross-sectional studies and the Newcastle-Ottawa scale16 for cohort studies (eMethods 2 in the Supplement). Case series on transmission were assessed using the quality assessment tool developed by Yanes-Lane et al.17 Two researchers (Q.L. and L.K.) performed the quality assessment independently. Disagreements were resolved by 2 other reviewers (W.J. and Y.W.). Outcomes of interest included the percentages of asymptomatic infections among the tested and the confirmed populations.

Statistical Analysis

We performed a meta-analysis to estimate the pooled percentage of asymptomatic infections among the tested and confirmed populations. Untransformed percentages and DerSimonian and Laird random-effects models18 were used to calculate the pooled percentage and its 95% CI. The heterogeneity among studies was assessed using I2 values.19 We performed subgroup analyses by study location (Africa, Asia, Europe, North America, and South America), countries’ development level (developed vs developing), study population (air or cruise travelers, close contact, community residents, health care workers or in-hospital patients, nursing home residents or staff, and pregnant women), publication period (June 2020 and earlier vs July 2020 and later), sample size for the tested population (1-99, 100-999, 1000-9999, and ≥10 000), sample size for the confirmed population (1-99, 100-499, and ≥500), study design (case series, case series on transmission, cohort studies, and cross-sectional studies), study quality (low, moderate, and high), MFR (0 to <0.5, 0.5 to <1.0, 1.0 to <1.5, and ≥1.5), and mean age (<20, 20-39, 40-59, and ≥60 years). Publication bias was assessed by funnel plot and the Egger regression test.20 We performed 3 sensitivity analyses to test the robustness of our results, by using the Knapp-Hartung adjustments21 to calculate the 95% CIs around the pooled effects, by excluding 3 studies with a tested population more than 200 000 and studies with low quality. Two-sided P < .05 indicated statistical significance. All analyses were performed using R, version 4.0.0 (R Project for Statistical Computing).

Results

We identified 2860 studies through database search and the reference lists of articles and reviews. Of these, 282 studies underwent full-text review. Ninety-five studies with information concerning the percentage of asymptomatic infections among the tested and confirmed populations were included in the final analysis12,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115 (Figure 1).

Figure 1. Flow Diagram of Study Selection.

Figure 1.

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Among these studies, 44 (46.32%) were cross-sectional studies, 41 (43.16%) were cohort studies, 7 (7.37%) were case series, and 3 (3.16%) were case series on transmission studies. Thirty-five studies (36.84%) were conducted in Europe; 32 (33.68%), in North America; and 25 (26.32%), in Asia. Seventy-four studies (77.89%) were conducted in developed countries. Thirty-seven studies (38.95%) were conducted among health care workers or in-hospital patients; 17 (17.89%), among nursing home residents or staff; 14 (14.74%), among community residents; 13 (13.68%), among pregnant women; 8 (8.42%), among air or cruise travelers; and 6 (6.32%), among close contacts. Twenty-one studies (22.11%) were published in June or before; 74 (77.89%), in July and after. Forty-nine studies (51.58%) had sample size of 100 to 1000. Fifty-three studies (55.79%) were assessed as low quality; 17 (17.89%), high quality; and 25 (26.32%), moderate quality (Table). For cross-sectional studies, low-quality studies were mostly those without random sampling or with 2 or more biases (selection bias, reporting bias, or detection bias). For cohort studies, low-quality studies were mostly those with 1 or more biases.

Table. Characteristics of the Studies Included for Meta-analysis.

Source Country Study design Time of publication Population group No. tested individuals No. confirmed individuals No. asymptomatic infections Quality
Abdelmoniem et al22 Egypt Cross-sectional January 2020 Health care workers or in-hospital patients 203 29 29 Low
Abeysuriya et al23 UK Cross-sectional September 2020 Pregnant women 180 7 6 Low
Akbarialiabad et al24 Iran Cross-sectional September 2020 Health care workers or in-hospital patients 1805 86 19 Low
Al-Qahtani et al25 Kingdom of Bahrain Cohort November 2020 Air or cruise travelers 2714 188 116 High
Al-Shamsi et al26 United Arab Emirates Cohort November 2020 Health care workers or in-hospital patients 109 32 6 Low
Arnold et al27 US Cross-sectional January 2021 Health care workers or/in-hospital patients 2882 103 38 Moderate
Arons et al12 US Cross-sectional April 2020 Nursing home residents or staff 76 48 27 Moderate
Aslam et al28 US Cohort January 2020 Health care workers or in-hospital patients 11 622 69 42 Low
Bayle et al29 France Cross-sectional January 2021 Nursing home residents or staff 241 32 24 Moderate
Bender et al30 US Cohort September 2020 Pregnant women 318 8 8 Moderate
Bianco et al31 US Cross-sectional May 2020 Pregnant women 155 24 24 Low
Blain et al32 US Case series July 2020 Nursing home residents or staff 113 44 8 Moderate
Blitz et al33 US Cohort August 2020 Pregnant women 382 71 45 Low
Blumberg et al34 US Cohort October 2020 Health care workers or in-hospital patients 1198 7 6 Low
Bosworth et al35 UK Cross-sectional July 2020 Health care workers or in-hospital patients 1282 53 16 Moderate
Cao et al36 China Cross-sectional November 2020 Community residents 9 865 404 300 300 High
Carroll et al37 Ireland Cohort October 2020 Close contact 4586 310 209 Moderate
Cattelan et al38 Italy Cohort August 2020 Health care workers or in-hospital patients 7595 395 109 Low
Cloutier et al39 Canada Cross-sectional August 2020 Community residents 330 6 6 Low
Corcorran et al40 US Cohort August 2020 Health care workers or in-hospital patients 25 10 4 Low
Deng et al41 China Case series on transmission October 2020 Close contact 347 27 1 High
Dora et al42 US Cross-sectional May 2020 Nursing home residents or staff 235 27 18 Low
Duan et al43 China Cross-sectional September 2020 Health care workers or/in-hospital patients 4729 4 4 Moderate
Figueiredo et al44 Portugal Cohort October 2020 Pregnant women 184 11 9 Low
Goldfarb et al45 US Cross-sectional May 2020 Pregnant women 757 20 9 Moderate
Graham et al46 UK Cross-sectional September 2020 Nursing home residents or staff 464 129 54 Moderate
Grechukhina et al47 US Cohort November 2020 Pregnant women 1567 141 44 High
Gruskay et al48 US Cohort June 2020 Health care workers or in-hospital patients 99 12 7 Low
Han et al49 China Cross-sectional June 2020 Community residents 29 299 18 18 Low
Harada et al50 Japan Cohort December 2020 Health care workers or in-hospital patients 1259 79 33 Low
Hcini et al51 France Cohort February 2020 Pregnant women 507 137 103 Low
Hoxha et al52 Belgium Cross-sectional July 2020 Nursing home residents or staff 280 427 8325 6244 Moderate
Hung et al53 China Case series September 2020 Air or cruise travelers 215 9 6 High
Ibrahim et al54 Indonesia Case series August 2020 Health care workers or in-hospital patients 4617 582 55 Low
Kennelly et al55 Ireland Cohort September 2020 Nursing home residents or staff 2968 1105 290 Low
Kessler et al56 Germany Cross-sectional December 2020 Health care workers or in-hospital patients 689 1 1 Moderate
Kimball et al57 US Cross-sectional April 2020 Nursing home residents or staff 76 23 13 Moderate
Kirshblum et al58 US Cohort July 2020 Health care workers or in-hospital patients 103 12 12 Low
Krüger et al59 Germany Cohort January 2021 Health care workers or in-hospital patients 6940 27 7 Low
Kwon et al60 South Korea Cross-sectional July 2020 Health care workers or in-hospital patients 2087 42 6 Low
LaCourse et al61 US Cohort May 2020 Pregnant women 230 13 1 Low
Ladhani et al62 UK Cohort September 2020 Nursing home residents or staff 518 158 97 High
Lan et al63 US Cross-sectional November 2020 Community residents 104 21 16 Moderate
Lavezzo et al64 Italy Cross-sectional July 2020 Community residents 2812 73 29 Moderate
Livingston et al65 UK Cohort October 2020 Health care workers or in-hospital patients 344 131 16 Moderate
Lombardi et al66 Italy Cohort June 2020 Health care workers or in-hospital patients 1573 139 28 Low
Ly et al67 France Cross-sectional November 2020 Nursing home residents or staff 1691 226 46 Moderate
Lytras et al68 Greece Cross-sectional April 2020 Air or cruise travelers 783 40 35 Low
Maechler et al69 Germany Cross-sectional December 2020 Community residents 4333 333 14 High
Marossy et al70 UK Cross-sectional September 2020 Nursing home residents or staff 2455 160 115 Moderate
Marschner et al71 Germany Cross-sectional July 2020 Health care workers or in-hospital patients 139 1 1 Low
Martinez-Fierro et al72 Mexico Cross-sectional October 2020 Close contact 81 34 5 Low
Massarotti et al73 Italy Cross-sectional August 2020 Pregnant women 333 7 6 Low
Mattar et al74 Caribbean Cross-sectional December 2020 Close contact 686 35 18 Low
Menting et al75 Germany Cross-sectional January 2020 Health care workers or in-hospital patients 1185 11 2 Low
Migueres et al76 France Cross-sectional September 2020 Health care workers or in-hospital patients 123 44 17 Low
Milani et al77 Italy Cross-sectional June 2020 Community residents 197 21 21 Moderate
Nishiura et al78 Japan Cross-sectional May 2020 Air or cruise travelers 565 13 4 Low
Ochiai et al79 Japan Cross-sectional June 2020 Pregnant women 52 2 2 Low
Olalla et al80 Spain Cross-sectional August 2020 Health care workers or in-hospital patients 498 2 2 Low
Olmos et al81 Chile Cross-sectional January 2021 Health care workers or in-hospital patients 413 14 14 Low
Park et al82 South Korea Cross-sectional April 2020 Community residents 1143 97 8 High
Park et al83 Korea Cohort December 2020 Air or cruise travelers 39 30 4 Low
Patel et al84 United States Cohort June 2020 Nursing home residents or staff 126 35 14 Low
Pavli et al85 Greece Case series on transmission September 2020 Air or cruise travelers 891 5 2 High
Petersen et al86 United Kingdom Cross-sectional October 2020 Community residents 36 061 115 88 Moderate
Puckett et al87 United States Cohort December 2020 Health care workers or in-hospital patients 227 2 2 Low
Ralli et al88 Italy Cohort December 2020 Community residents 298 12 9 Low
Rashid-Abdi et al89 Sweden Cohort November 2020 Health care workers or in-hospital patients 131 21 1 Low
Ren et al90 China Cohort February 2021 Air or cruise travelers 19 398 384 3103 1749 High
Rincón et al91 Spain Cohort September 2020 Health care workers or in-hospital patients 192 36 14 Low
Roxby et al92 United States Cohort May 2020 Nursing home residents or staff 80 3 2 Low
Sacco et al93 France Cohort November 2020 Nursing home residents or staff 179 63 12 Low
Santos et al94 Portugal Cross-sectional December 2020 Health care workers or in-hospital patients 8037 211 47 Low
Scheier et al95 Switzerland Cross-sectional February 2021 Health care workers or in-hospital patients 2807 68 8 High
Shah et al96 US Case series July 2020 Health care workers or in-hospital patients 625 1 1 Low
Shi et al97 US Cohort October 2020 Nursing home residents or staff 389 146 66 Moderate
Singer et al98 US Case series October 2020 Health care workers or in-hospital patients 4751 18 10 High
Tang et al99 China Cross-sectional July 2020 Health care workers or in-hospital patients 1027 52 13 High
Tang et al100 US Cohort November 2020 Nursing home residents or staff 1970 752 424 High
Temkin et al101 Israel Cross-sectional October 2020 Health care workers or in-hospital patients 522 1 1 Low
Trahan et al102 Canada Cohort November 2020 Pregnant women 803 41 11 Low
Tsou et al103 China Case series November 2020 Community residents 17 935 100 10 Moderate
van Buul et al104 The Netherlands Cohort Decem ber 2020 Nursing home residents or staff 839 25 6 High
Varnell et al105 US Cohort January 2021 Health care workers or in-hospital patients 281 24 9 Moderate
Wadhwa et al106 US Cohort December 2020 Community residents 172 19 12 Moderate
Wi et al107 South Korea Case series July 2020 Community residents 17 400 111 25 High
Wood et al108 Indiana Cross-sectional August 2020 Community residents 511 1 1 Low
Yamahata et al109 Japan Cross-sectional May 2020 Air or cruise travelers 3711 696 410 Moderate
Yassa et al110 Turkey Cohort July 2020 Pregnant women 296 23 12 Low
Yau et al111 Canada Cohort July 2020 Health care workers or in-hospital patients 330 22 12 Low
Yousaf et al112 US Cohort July 2020 Close contact 195 47 6 Low
Zhang et al113 China Case series on transmission April 2020 Close contact 8437 25 3 High
Zhang et al114 China Cohort September 2020 Health care workers or in-hospital patients 8553 235 21 Low
Zhao et al115 China Cohort August 2020 Health care workers or in-hospital patients 1060 160 38 Low
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Percentage of Asymptomatic Infections Among the Tested Population

Ninety-five studies were included in the meta-analysis for the percentage of asymptomatic infections among the tested population, covering 29 776 306 tested individuals, among whom 11 516 had asymptomatic infections. The pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.23%-0.27%), with high heterogeneity among studies (I2 = 99%; P < .001) (eFigure 1 in the Supplement).

Among tested individuals in different study populations, the pooled percentage of asymptomatic infections was 4.52% (95% CI, 4.15%-4.89%) in nursing home residents or staff, 2.02% (95% CI, 1.66%-2.38%) in air or cruise travelers, 2.34% (95% CI, 1.89%-2.78%) in pregnant women, 1.46% (95% CI, 1.05%-1.88%) in close contacts, 0.75% (95% CI, 0.60%-0.90%) in health care workers or in-hospital patients, and 0.40% (95% CI, 0.18%-0.62%) in community residents. The pooled percentage of asymptomatic infections was 0.90% (95% CI, 0.87%-0.93%) in Europe, 0.47% (95% CI, 0.39%-0.54%) in North America, and 0.05% (95% CI, 0.04%-0.07%) in Asia. The pooled percentage was higher in developed countries (0.70% [95% CI, 0.67%-0.73%]), studies published in July or later (0.29% [95% CI, 0.27%-0.31%]), studies with a sample size of less than 100 (6.74% [95% CI, 4.69%-8.80%]), and cohort studies (2.98% [95% CI, 2.68%-3.29%]). In studies with MFR of 0.5 to less than 1.0, the pooled percentage was higher (3.91%; [95% CI, 3.14%-4.68%]). The pooled percentage was higher when the mean age of the study population was 60 years or older (3.69% [95% CI, 2.99%-4.39%]) (Figure 2).

Figure 2. Percentage of Asymptomatic Infections Among the Tested Population by Subgroups.

Figure 2.

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Includes 29 776 306 tested individuals, among whom 11 516 had asymptomatic infections.

Percentage of Asymptomatic Infections Among the Confirmed Population

Among 95 studies, 18 were excluded because that the percentage of asymptomatic infections among the confirmed population was 100%.22,30,31,36,39,43,49,56,58,71,77,79,80,81,87,96,101,108 The remaining 77 studies were included in the meta-analysis for the percentage of asymptomatic infections among the confirmed population,12,23,24,25,26,27,28,29,32,33,34,35,37,38,40,41,42,44,45,46,47,48,50,51,52,53,54,55,57,59,60,61,62,63,64,65,66,67,68,69,70,72,73,74,75,76,78,82,83,84,85,86,88,89,90,91,92,93,94,95,97,98,99,100,102,103,104,105,106,107,109,110,111,112,113,114,115 covering 19 884 individuals with confirmed COVID-19, among whom 11 069 had asymptomatic infections. The pooled percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 33.50%-47.50%), with high heterogeneity among studies (I2 = 99%; P < .001) (eFigure 2 in the Supplement).

Among the confirmed population, the pooled percentage of asymptomatic infections was 54.11% (95% CI, 39.16%-69.05%) in pregnant women, 52.91% (95% CI, 36.08%-69.73%) in air or cruise travelers, 47.53% (95% CI, 36.36%-58.70%) in nursing home residents or staff, 39.74% (95% CI, 24.50%-54.98%) in community residents, 30.01% (95% CI, 21.13%-38.88%) in health care workers or in-hospital patients, and 26.94% (95% CI, 8.50%-45.38%) in close contacts. The pooled percentage of asymptomatic infections was 46.32% (95% CI, 33.47%-59.16%) in North America, 44.18% (95% CI, 32.87%-55.50%) in Europe, and 27.58% (95% CI, 13.60%-41.57%) in Asia. The pooled percentage was higher in developed countries (43.51% [95% CI, 35.59%-51.44%]), studies published in June or earlier (43.68% [95% CI, 27.87%-59.50%]), studies with sample size of 500 or greater (47.06% [95% CI, 26.22%-67.90%]), and cross-sectional studies (44.47% [95% CI, 33.54%-55.40%]). The pooled percentage was slightly lower for cohort studies (40.96% [95% CI, 31.18%-50.74%]). Among studies with MFR of 1.0 to less than 1.5, the pooled percentage was higher (55.09% [95% CI, 27.64%-82.53%]). The pooled percentage was higher when the mean age of the study population was younger than 20 years (60.21% [95% CI, 24.51%-95.91%]) or 20 to 39 years (49.49% [95% CI, 33.48%-65.50%]) (Figure 3).

Figure 3. Percentage of Asymptomatic Infections Among the Confirmed Population by Subgroups.

Figure 3.

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Includes 19 884 individuals with confirmed COVID-19, among whom 11 069 had asymptomatic infections.

Sensitivity Analysis and Publication Bias

After using the Knapp-Hartung adjustments, the pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.11%-0.39%), and the 95% CI of the pooled percentage became slightly larger (eFigure 3 in the Supplement). The percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 34.94%-46.07%), and the 95% CI of the pooled percentage became slightly narrower (eFigure 4 in the Supplement).

After excluding 3 studies with tested populations of more than 200 000,36,52,90 the pooled percentage of asymptomatic infections among the tested population was 1.61% (95% CI, 1.47%-1.76%), which was higher than the original results. The percentage of asymptomatic infections among the confirmed population was 39.37% (95% CI, 33.86%-44.87%), which was slightly lower than the original results. After excluding 53 low-quality studies, the pooled percentage of asymptomatic infections among the tested population was 0.24% (95% CI, 0.23%-0.26%), and the percentage of asymptomatic infections among the confirmed population was 41.71% (95% CI, 31.89%-51.53%). Both percentages were similar to the original results.

Funnel plots are shown in Figure 4. Egger regression tests for the percentage of asymptomatic infections among the tested population (z = 43.1725; P < .001) and for the percentage of asymptomatic infections among the confirmed population (z = 2.3846; P = .02) indicated that there might be publication bias.

Figure 4. Funnel Plots Based on the Percentage of Asymptomatic Infections.

Figure 4.

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Includes 29 776 306 tested individuals, among whom 11 516 had asymptomatic infections and 19 884 individuals with confirmed COVID-19, among whom 11 069 had asymptomatic infections. Funnel plot asymmetry indicated possible publication bias.

Discussion

In this meta-analysis, we found that the pooled percentage of asymptomatic infections among the tested population was 0.25% (95% CI, 0.23%-0.27%), and the pooled percentage of asymptomatic infections among the confirmed population was 40.50% (95% CI, 33.50%-47.50%). At present, there are only a few meta-analyses for the percentage of asymptomatic infections among the tested population. We found that the percentage of asymptomatic infections was highest among the tested population in nursing homes and lowest among community residents. Because the percentage of asymptomatic individuals varies as a function of community prevalence, it was not available in all studies. This might be a potential driver of heterogeneity across studies. Furthermore, the percentages of asymptomatic infections among the tested population were different between studies conducted in different locations. Studies in Asia had the lowest percentage, whereas studies in other locations had higher percentages. This lower percentage in Asia might be related to the large city-wide SARS-CoV-2 nucleic acid screening program in China.36 In the sensitivity analyses, we found that the pooled percentage of asymptomatic infections among the tested population was higher than the original results after excluding studies with large sample sizes. This indicated that studies with different sample sizes were very heterogeneous. Owing to severe outcomes among older patients with COVID-19, more studies were conducted among nursing home residents or staff. Thus, asymptomatic individuals were more likely to be tested among this population. As more and more countries conducted expanded screening, studies concerning the percentage of asymptomatic infections among the general population would increase in the future.

In this study, the pooled percentage of asymptomatic infections among the confirmed population was 40.50%. The pooled percentage of asymptomatic infections was 40.96% among cohort studies, which was slightly lower than that among cross-sectional studies (44.47%). The patients who developed symptoms later were mistakenly classified as having asymptomatic infection in cross-sectional studies because the observation time was not long enough.14 Thus, the percentage of asymptomatic infections was lower in cohort studies, because some patients with presymptomatic findings were identified during follow-up. There were limited case series of great interest in the first months of the pandemic; however, these studies mostly traced and tested limited contacts, which contributed limited value to the evidence of the percentage of asymptomatic infections.17 Several meta-analyses concerned the percentage of asymptomatic infections among the confirmed population. Chen et al5 conducted a meta-analysis that included 104 published studies and preprints before May 13, 2020. They found that the percentage of asymptomatic individuals among those with COVID-19 was 13.34% (95% CI, 10.86%-16.29%). Unlike our study, Chen et al5 searched a Chinese database. Thus, the percentage of Chinese studies was higher in their study than in the present study. He et al14 searched PubMed and Embase before May 20, 2020, and included 41 published studies. More than 50% of the studies were from China, and the pooled percentage of asymptomatic infection was 15.6% (95% CI, 10.1%-23.0%). In our study, we only included published studies. The percentage of countries excluding China was higher than the previous meta-analysis.14 This might be the reason for the higher percentage of asymptomatic infections found in our study compared with studies conducted by Chen et al5 and He et al.14 Another meta-analysis conducted by Yanes-Lane et al17 included published studies and preprints before June 22, 2020. After quality assessment, 28 studies were of high or moderate quality and were included in the meta-analysis. The percentage of asymptomatic infection among persons with confirmed COVID-19 varied among different study populations, with the highest observed in obstetric patients (95% [95% CI, 45%-100%]).

In our study, the percentage of asymptomatic infections among the confirmed population was 54.11% in pregnant women and 52.91% in air or cruise travelers. The percentage was 47.53% in nursing home residents or staff. This finding of a high percentage of asymptomatic infections among air or cruise travelers suggests that screening and quarantine on airport arrival is important for reducing community transmissions, especially in countries without local transmission.3,25 In addition, we found that the percentage of asymptomatic infections among the tested population was relatively low among community residents. However, the percentage of asymptomatic infection among confirmed individuals was 39.74% in communities. These findings suggest that asymptomatic infections might contribute to the transmission of SARS-CoV-2 within the community. To prevent further transmission in communities, asymptomatic individuals among the general population should be tested. If resources are limited, workers in specific industries such as air transportation should be routinely tested. In addition, we found that approximately one-third of individuals with confirmed COVID-19 were asymptomatic among health care workers or in-hospital patients. Because asymptomatic health care workers might contribute to disease spread in and out of hospitals, surveillance of asymptomatic individuals is important for infection control and transmission reduction in health care settings and community.116,117 Meanwhile, hand hygiene and personal protective equipment were necessary for hospital visitors.117 A previous study showed that most asymptomatic patients belong to younger groups,3 which was consistent with the findings of our study. The percentage of asymptomatic infections was higher among groups younger than 39 years than in other age groups, possibly because the young adults were more likely to show only mild or moderate clinical symptoms.5 This indicated that young adults who often presented mild or no symptoms were a potential source of transmission in the community.

In the meta-analysis, we included studies published before February 3, 2021, providing the most updated pooled percentage of asymptomatic infections among tested and confirmed populations. We included countries in Africa, Asia, Europe, North America, and South America and estimated the percentage of asymptomatic infections for different populations. Our results could raise awareness among the public and policy makers and provide evidence for prevention strategies.

Limitations

This study has several limitations. First, we did not include preprints and therefore may have missed some relevant studies; however, we thought that the results of published studies were more reliable. Second, some relevant articles written in Chinese may not be included because we did not search Chinese literature databases such as China National Knowledge Infrastructure. Third, most studies did not follow up to identify presymptomatic and covert infections. Future studies should evaluate the percentage of these 2 types of asymptomatic infection among the confirmed population. Fourth, most studies were conducted in a specific population; thus, our findings might not be generalizable to the general population. Fifth, the heterogeneity between studies was high, which might be related to different study location, period, population, and sample size. Sixth, the Egger regression test suggested potential publication bias in this study. Because studies that did not detect asymptomatic infections were less likely to be published, our pooled percentage of asymptomatic infections might be overestimated.

Conclusions

In this systematic review and meta-analysis, we found that the pooled percentage of asymptomatic SARS-CoV-2 infections among the tested population was 0.25%. Among the confirmed population, 40.50% of individuals had asymptomatic infections. The high percentage of asymptomatic infections highlights the potential transmission risk of asymptomatic infections in communities. Screening for asymptomatic infection is required, especially for countries and regions that have successfully controlled SARS-CoV-2. Asymptomatic infections should be under management similar to that for confirmed infections, including isolating and contact tracing.

Supplement.

eMethods 1. Search Strategies

eMethods 2. Study Quality Assessments

eFigure 1. Percentage of Asymptomatic Cases Among the Tested Population

eFigure 2. Percentage of Asymptomatic Cases Among the Confirmed Population

eFigure 3. Percentage of Asymptomatic Cases Among the Tested Population by Subgroups, Using the Knapp-Hartung Adjustments

eFigure 4. Percentage of Asymptomatic Cases Among the Confirmed Population by Subgroups, Using the Knapp-Hartung Adjustments

Click here for additional data file. (1.4MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplement.

eMethods 1. Search Strategies

eMethods 2. Study Quality Assessments

eFigure 1. Percentage of Asymptomatic Cases Among the Tested Population

eFigure 2. Percentage of Asymptomatic Cases Among the Confirmed Population

eFigure 3. Percentage of Asymptomatic Cases Among the Tested Population by Subgroups, Using the Knapp-Hartung Adjustments

eFigure 4. Percentage of Asymptomatic Cases Among the Confirmed Population by Subgroups, Using the Knapp-Hartung Adjustments

Click here for additional data file. (1.4MB, pdf)

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