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 Table of Contents  
Year : 2021  |  Volume : 10  |  Issue : 1  |  Page : 11-16

COVID-19: The enigma of asymptomatic carriers and silent transmission

Department of Public Health and Community Medicine, Faculty of Medicine, Mansoura University, Mansoura, Egypt

Date of Submission11-Jul-2020
Date of Decision07-Aug-2020
Date of Acceptance10-Sep-2020
Date of Web Publication2-Feb-2021

Correspondence Address:
Abdel-Hady El-Gilany
Faculty of Medicine, Mansoura University, Mansoura 35516
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijhas.IJHAS_135_20

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The presence of asymptomatic carriers is well known in the COVID-19. These carriers play a significant role in the disease transmission. Little is known about its magnitude, types, detection, and management. This narrative review highlights all these aspects of asymptomatic COVID-19 carriers. Much is still unknown about asymptomatic carriers in COVID-19 and its contribution in the silent spread of infection. A literature of three electronic databases with prespecified search terms was done to include the status quo data on asymptomatic carriers in COVID-19 infection. This narrative review will be useful for epidemiologists, clinicians, policymakers as well as the public in general.

Keywords: Asymptomatic carrier, COVID-19, presymptomatic carrier-silent spread

How to cite this article:
El-Gilany AH. COVID-19: The enigma of asymptomatic carriers and silent transmission. Int J Health Allied Sci 2021;10:11-6

How to cite this URL:
El-Gilany AH. COVID-19: The enigma of asymptomatic carriers and silent transmission. Int J Health Allied Sci [serial online] 2021 [cited 2023 Mar 27];10:11-6. Available from: https://www.ijhas.in/text.asp?2021/10/1/11/308581

  Introduction Top

COVID-19 is a novel human coronavirus identified in China in December 2019. Much is unknown about its basic epidemiology, including the asymptomatic carriers, which is important for prevention and control activities. The key to prevent silent spread of the virus is to understand the symptomless carriers. Many scholars believe that a large number of asymptomatic carriers are responsible for rapid spread of COVID-19.[1],[2],[3],[4] Asymptomatic carriers or silent spreaders refer to people who tested positive for the coronavirus but develop no symptoms. They are infectious and pose a risk of spreading infection to others. The prevalence of asymptomatic carriers is a challenge aspect of the coronavirus pandemic. Many studies reported asymptomatic people seems to have the same viral load as cases with symptoms. Ultimately, testing blood for antibodies in community surveys is the only way to really find out the prevalence of asymptomatic COVID-19 carriers. These tests are not available nowadays and being developed in several countries.[3],[5],[6],[7] The lack of a reliable diagnostic test for the timely detection of asymptomatic carriers is a major limitation to control the spread of the disease.

  Search Strategy Top

This is a status quo review that included the current research related to COVID-19 carriers. A literature search was conducted with the following three electronic databases: PubMed, Scopus, and Google Scholar. Search terms included “COVID-19,” “Types of carriers,” “prevalence of asymptomatic carriers” “sites of viral carriage,” “Asymptomatic carrier,” “detection of carriers,” “importance of carriers,” and “control of asymptomatic carriers,” and these terms were used in different combinations. All articles and books published in English were read and assessed for inclusion.

  Study Rationale Top

Despite of their epidemiological significance, the effects of carriers on the transmission dynamics of COVID-19 received inadequate attention in the mathematical research modeling. Comprehension of the natural history of carriers and their contribution to spread of infection is a prerequisite for formulating effective measures for prevention and control. The ratio of in-apparent to apparent infection in COVID-19 is definitely unknown. There is no public data about the number of asymptomatic carriers in most of the countries. Understanding the natural history of asymptomatic or presymptomatic infection will help in personalized risk stratification and reveal the protective immunity, whether artificial or natural.

  Classification and Types of Carriers Top

For some infectious diseases, there are subjects who can transmit the infection but do not show any symptoms and they are called carriers or asymptomatic carriers and the term “symptomatic infection” concerns those with observable clinical manifestation.[8] Infectious disease carriers should be differentiated from a related term; genetic carriers who carry the disease on their recessive genes and only pass it to their children and are not contagious.[9]

Epidemiologically, a carrier is an animal or person that harbors and spreads an infectious agent without apparent clinical manifestations. This carrier may by immune person with an infection that is in-apparent throughout its course (also known as passive carrier, healthy carrier, or subclinical case), or during the incubation or convalescence period of an individual with a clinically recognizable disease (called incubatory or convalescent carrier; respectively). In both circumstances the carriage state may last for short or long duration (transient/temporary or chronic carrier; respectively) or intermittent carrier who discharges organism at intermittent intervals.[10]

Epidemiologists have described three types of silent spreaders of COVID-19, i.e., asymptomatic, presymptomatic, and convalescent carriers:[2],[3],[11],[12],[13],[14],[15],[16],[17],[18],[19]

Asymptomatic carriers

Asymptomatic carriers are people without any symptom at all while incubating the virus in their bodies. They have no symptoms or clinical signs throughout the 14-day quarantine. They transmit the virus to a lot of people and fuel the pandemic. They may become immune despite they never have any symptoms during their infections. However, currently, it is difficult to identify truly asymptomatic carriers and their immune status.

Presymptomatic carriers (incubatory carriers)

These people do not develop any symptoms for almost a week after contracting the novel coronavirus. They later display symptoms such as coughing, fever, and exhaustion. The levels of the virus are high in respiratory discharges and spread infection during the presymptomatic period that can last from days to weeks prior to the characteristic symptoms of COVID-19.

Postsymptomatic (convalescent/postconvalescence) carriers

Individuals with a clinically recognizable disease who spread infection for different periods after disappearance of symptoms. The agent is not completely eliminated due to inadequate treatment or deficient immune response resulting in the carrier state.

A related term is the paradoxical carrier is a person who acquires the organism from another carrier. Asymptomatic and presymptomatic carriers should be differentiated from unreported and missed cases due to inadequate case finding.

  Sites of Carriage and Exit Top

This differs from a causative agent to another. The COVID-19 virus was detected in all parts of the respiratory tract and find exit with respiratory discharges. Furthermore, the virus was detected in intestine and some patients develop gastrointestinal symptoms. The initial investigations detected the virus in stool of some cases; though this is not a main route of spread during the outbreak. There is a low risk of catching COVID-19 from the feces of an infected person.[11],[12] Hands and surfaces can be contaminated with the virus and constitute a potential route of oral transmission.[20]

  Magnitude of Asymptomatic Carriers Top

Recent epidemiologic models found that asymptomatic or mild cases that pass undetected and not documented ranged from 5% to 85% of total infections in different studies. This variation is related to age group, study methods, study setting, extent of screening in the exposed and contacts, as well as duration of follow-up.[5],[13],[17],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38]

However, most of these studies typically estimated the prevalence of asymptomatic carriers only at single points in time; other follow-up studies have shown that a large proportion of asymptomatic people with initially positive test develop symptoms later on.[39]

It was estimated that up to 86% of all infections were not documented and their transmission rate per person was 55% the transmission rate of documented infections. The undocumented infections were the source of 79% of the documented cases because of their greater numbers.[40] The actual prevalence of asymptomatic and mild infections will remain unknown until a cheap serological test, detecting the viral antibodies signaling previous infection, is available for routine use.[41]

  Detection of Carriers Top

It is usually possible to identify asymptomatic carriers. The Centers for Disease Control estimates that coronavirus can be picked up by a conventional test around 1–2 days before symptoms appear. This means people may have enough of the virus in their bodies to be detected by laboratory tests, but they might not feel sick yet.[42] COVID-19 carriers can be detected by polymerase chain reaction (PCR) for swabs from respiratory tract (nose, naso-pharynx, and oropharynx) or bronchial aspirate, as well as stool culture. Currently, no serologic detection is available.[11] However, caution should be taken as detected RNA may be a remnant of nonbiologically active virus not capable of transmitting infection.[39] As other corona viruses, it is possible most people are developing antibodies after recovery from COVID-19 that will offer some protection from reinfection,[43] however, no antibodies test was available yet.

  Importance of Carriers Top

The carrier is unapparent and spreads infection for different periods.[10] Chen et al.[44] raised the alarm of the existence of the transmissibility of the asymptomatic COVID-19 cases. However, He et al.[45] concluded that the relative transmissibility of asymptomatic persons could be smaller than that of the symptomatic patients. Furthermore, there is a high level of viral shedding in the upper respiratory tract among presymptomatic patients.[46],[47],[48] Presymptomatic transmission occurred 1–3 days before symptoms development.[49] It has been inferred that presymptomatic spread contributed up to 62% of transmissions even in the presence of control measures.[50]

Epidemiologically, carriers are more important than cases as their number may outnumber cases, they are difficult to recognize as they have no symptoms and unaware of being infected, they move freely (cases are restricted) and chronic carriers, if present, re-introduce infection, and contribute to endemicity. It is possible that asymptomatic carriers in COVID-19 may be responsible for what is known as community spread or transmission. Researchers failed to relate a large number of confirmed cases through the usual chains of transmission. Furthermore, there is an increasing number of positives in routine testing of respiratory samples for other indications.[51] Asymptomatic carriers generate speech droplets that are considered to be the mode of infection transmission.[52] However, the seriousness of a carrier as a source of infection depends on his occupation, dwelling conditions, and personal habits.[36]

The vague nature of asymptomatic carrier is closely related to the prevention and control of COVID-19.

Identifying asymptomatic carriers is useful to highlight those with higher risk of poorer outcomes from a given treatment or procedure, allowing possible deferral for care or amend treatment plans (e.g., hold immune suppressing treatment until infection resolved), and conserve the personal protective equipment to those who in need.

Light cases and asymptomatic carrier are not counted as COVID-19 cases and they will exacerbate the spread of infections. Those who develop symptoms should be reclassified as a confirmed case.

The incidence data only include symptomatic cases and the true magnitude of asymptomatic carrier remains unknown.

Asymptomatic carriers should be included in calculation of the basic reproduction number (R0). Kalajdzievska and Li[9] formulated a model for prediction R0 including the susceptible, carrier, symptomatically infectious and removed classes and the model yields a much higher R0.

With lack of direct evidence of asymptomatic carriers it is hard to interpret the epidemiological record with uncertainties about the carrier state.[53],[54] For example, estimating the duration of asymptomatic carrier is difficult due to limited data on exact time of catching infection and of subsequent viral clearance.[55]

The existence of asymptomatic carriers compromise the control efforts that depends on detecting infectious cases, for example, large-scale monitoring, isolating, and treating infectious cases.[56]

Asymptomatic carriers affect the usefulness of interventions targeting susceptible subjects because of difficultly in distinguishing between the two groups.[57]

Accidental treatment of these carriers could result in the emergence and spread of drug-resistant agents.[58],[59]

  Prevention and Control of Asymptomatic Carriers Top

The extent of asymptomatic or presymptomatic spread is definitely unknown. Asymptomatic surveillance (testing people without symptoms) is an important strategy to collect data on how COVID-19 is spread in the community. It should be stressed that the result is instant that is a negative person may develop symptoms or turn positive at a later time and needs re-testing.

The following are measures to be taken for prevention and control of asymptomatic carriers:[15],[28],[50],[60]

  1. Screening and detection of asymptomatic carriers should be stepped up by testing: (1) close contacts of confirmed cases; (2) people involved in investigations of clusters; (3) exposed people during tracing of the infection source; (4) people with history of travel or residence in areas endemic with COVID-19; and (5) during opportunistic screening and epidemiological investigation

  2. There is an argue for extending COVID-10 testing to include asymptomatic persons in high-risk and dangerous settings and enforce the general public to use face masks when in crowded outdoor or indoor spaces.[61] Hu et al.[62] highlighted the importance of contact tracing and longitudinal surveillance by PCR. Further isolation and continuous PCR testing are recommended to the patients after discharge. As serological assays are still not available, pooled PCR testing of a random community sample could allow for rapid assessment of the prevalence of clinical, missed and subclinical cases in the community[63],[64]

  3. Standardize reporting of asymptomatic carriers by all types and levels of health-care facilities
  4. Proper management of asymptomatic persons and they should be quarantined for 14 days or negative two successive swabs. Those who develop clinical manifestations of COVID-19 during this period should be reclassified as confirmed cases and given the standard treatment. A single center study found that asymptomatic COVID-19 infections have longer treatment cycle than moderate types of confirmed cases[65]
  5. Contacts of asymptomatic persons should be put under medical observation for 14 days.
  6. Health education and counseling the asymptomatic carriers and their contacts to follow the routine preventive measures
  7. All data should be available for research on epidemiology, infectivity, and transmission of asymptomatic carriers.

  Conclusions Top

There is an epidemiologic uncertainty of the possible spread of the virus by asymptomatically or presymptomatic-infected persons together with the lack of effective drugs to eliminate this carrier state. It is a challenge to consider the possible presence of a large number of infectious COVID-19 carriers without symptoms. Because they are symptomless, they are not a part of any prevention and treatment measures. Further specific models for COVID-19 infection and more data are needed to be employed to predict future disease load. Large scale contact tracing as well as community surveys are essential to assess the magnitude of the asymptomatic carriers. Further research is needed to find out the percentage of carriers, their types, duration of carriers states, mode of exit, their epidemiologic significance, and testing drugs for clearance of the carriage state. There is a need to develop and apply a standard protocol for the management of asymptomatic carriers of COVID-19.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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Meera Karunakaran, Jayasree Chandrasekharan Nair Saradamma
Journal of Evolution of Medical and Dental Sciences. 2021; 10(45): 3920
[Pubmed] | [DOI]


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