There are many cases where asymptomatic or presymptomatic people have transmitted the disease to a mass number of people around them. Such cases lead to questions as to how the disease was transmitted as many asymptomatic or presymptomatic people do not exhibit the symptoms necessary to propel the larger droplets to reach a mass number of people. However, if the aerosol pathway is considered, it can lead to some insight as many studies support that aerosol droplets are associated with normal breathing and talking, and occasional coughing of healthy individuals predominate.

Transmission of SARS‐CoV‐2 from Aysmptomatic People

Previous reports indicate that asymptomatic transmission is possible, but since they’re hard to track, this poses a significant challenge to infection control practices.

University of Michigan - Ann Arbor

Smith College

Several studies have shown that COVID-19 can be transmitted by individuals who do not show any symptoms, or who are "asymptomatic". Because asymptomatic individuals are not routinely tested, the prevalence of asymptomatic infection is not well-defined.

Asymptomatic COVID-19 Infection

COVID-19 is spread through person-to-person transmission. The virus spreads mainly through close personal contact (such as when people are within 6 feet of one another) and through the respiratory droplets of someone infected with COVID-19.  Aerosol transmission is also likely, but the relative contribution of these smaller respiratory particles is not completely understood. While less common, spreading can also occur when people touch contaminated objects or surfaces and then their faces, eyes, or mouth. 

Transmission of COVID-19

Al-Tawfiq, J. (2020). Asymptomatic coronavirus infection: MERS-CoV and SARS-CoV-2 (COVID-19). Travel Medicine And Infectious Disease, 101608. doi.org/10.1016/j.tmaid.2020.101608
https://www.sciencedirect.com/science/article/pii/S1477893920300752?via%3Dihub


Asymptomatic coronavirus infection: MERS-CoV and SARS-CoV-2 (COVID-19)

Implications of Asymptomatic COVID-19 Transmission

- China
- Germany
- Japan
- USA
- Singapore

Asymptomatic COVID-19 Infection Categorized by Country

- Travel Medicine and Infectious Diseases References
-Science China Life Sciences
-Journal of American Medical Association (JAMA)
- New England Journal of Medicine (NEJM)
- Euro Surveillance
- Morbidity and Mortality Weekly Report (MMWR)
- Nature Medicine

Asymptomatic COVID-19 Infection References by Journal

- Viral shedding
- IgG levels
- Low white blood cell count (lymphopenia)
- Low blood platelet count (thrombocytopenia)
- Chest CT focal ground-glass opacities
- Chest CT stripe shadows

Clinical and Immunological Results of Asymptomatic COVID-19 Infection

Due to the SARS-CoV-2 viral load being similar between asymptomatic and symptomatic healthcare workers with COVID-19, asymptomatic healthcare workers donning less PPE may be a risk factor for other colleagues, patients, and the community.

Healthcare Workers with Asymptomatic COVID-19 Infection

In addition to symptomatic screening, additional control/detection methods are routine rRT-PCR testing, conservative guidelines for reopening, and periodic testing are important for epidemiological evaluations. Facilities using these methods include nursing homes and other congregate living situations such as prisons, mental health facilities, and homeless shelters.

Additional control/detection methods to prevent asymptomatic transmission

- Fever (when high temperature is measured)
- Laboratory confirmation of illness via viral testing
- Radiological lung abnormalities such as ground-glass chest and stripe shadowing


Clinical Characteristics of Asymptomatic COVID-19 Patients

Studies have shown that even though asymptomatic people have the same viral load, they have increased levels of lymphocytes (CD4+cells) as compared to symptomatic people, which may lead to them being asymptomatic. Due to the presence of increased lymphocytes, they also clear the viral particles faster and show lower risk of long term complications. 



Role for lymphocytes in asymptomatic COVID-19 infection

The reproductive/reproduction number of COVID-19 is estimated to be between 2 and 2.5 as of March 6, 2020. 

Reproductive Number of COVID-19

- American Association for the Advancement of Science

- American Veterinary Medical Association (AVMA)

- Asian Pacific Journal of Tropical Medicine (APJTM)

- BioMed Research International

- BioRxiv (Preprint)

- Emerging Infectious Diseases

- Environmental Research 

- Forensic Science International: Reports

- Infectious Disease Modeling 

- Infectious Diseases Society of America (Clinical Infectious Disease)

- Jama Network Open

- Journal of Travel Medicine

- Nature Communications

- Risk Analysis

- Science of the Total Environment

- Social Science Research Network (SSRN)

- The International Journal of Infectious Diseases  

- The Journal of the American Medical Association (JAMA)

- The Lancet

- The Lancet Infectious Diseases 

- The New England Journal of Medicine

- Travel Medicine and Infectious Diseases

- Veterinary Quarterly

- Virology Journal 

- World Journal of Clinical Cases




Transmission of COVID-19 References by journal

Cluster transmission has been noted in many COVID-19 studies
It is characterized by infections that are concentrated within a given area, often within families or social circles 

Cluster Transmission of COVID-19

- Asia
- Africa 
- North America
- South America
- Antarctica
- Europe
- Australia

Classification of studies on COVID-19 Transmission by Continents

Indirect Transmission of COVID-19

- Super spreading events

- Climate

- Season

- Location

- UV & sunlight on the virus

- Virus concentrations in wastewater 

Factors that affect the transmission of COVID-19

Live animal food markets carry a great risk of transmitting zoonotic diseases compounded by the lack of safe and regulated hygienic practices. The Huanan South Seafood Market in Wuhan, China which sells wild exotic and domesticated live animals for consumption, was identified to be the epicenter of the COVID-19 pandemic. Bats are implicated to be the reservoir hosts of the virus, as SARS-CoV-2 is 96% genetically identical to bat coronavirus. Pangolins are also a possible host, as the RBD in the S protein show similarity to Pangolin-CoV. 

Any infections in animals with COVID-19 should be reported to the World Organization for Animal Health (OIE). The CDC reports that a number of pets worldwide are confirmed positive for COVID-19, though the risk of animal-human transmission in these cases are yet unclear.

Zoonotic Transmission of COVID-19

In a comprehensive case study, Vivanti et al. report the first transplacental transmission of SARS-CoV-2 with clinical manifestation in the newborn, consistent with neurological signs and symptoms of the virus. 

Transplacental Transmission of COVID-19

Determining this has critical implications: 

If primarily spread via droplets, then masks, distancing rule (6ft), etc. are effective in preventing transmission. Conversely, if primarily spread via aerosols, then preventative measures become less effective and thus need to be modified. 

Examining the source of transmission for both types of respiratory particles are crucial.



Aerosol versus Droplet Debate

Transmission of COVID-19 references by organization

Current data shows that infection rates of SARS-CoV-2 are low among children. It then can be said that they do not play a prominent role in the disease transmission of COVID-19 during the pandemic. Although the actual infection rate cannot be known due to under-reporting and a lack of available results from seroepidemiological studies, there are numerous findings against an increased risk of infection from children and findings that show a lower rate of infection and of contagion especially in children under the age of 10. It was also found that the transmission of SARS-CoV-2 is typically from an infected adult to a child and that a lack of evidence remains for the transmission of infection from an infected child to an adult. The cause of this lower virus transmission rate is still unknown, which calls for further investigations. 

Role of Children In Transmission

- Genomic Evidence of In-Flight Transmission of SARS-CoV-2 Despite Predeparture Testing
-  In-flight transmission of SARS-CoV-2: a review of the attack rates and available data on the efficacy of face masks

Air travel and COVID-19 transmission 

SARS-CoV-2 is a branch of the SARS-Cov family. It is spherical and enveloped with club-shaped spikes on the surface. Thus, it has the shape of a solar corona. Inside the nucleoplasm, the virus has single-strand RNA, which stores the genetic information of this virus. Due to the property of protein and RNA, the SARS-CoV-2 should be sensitive to temperature.

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