- Medical procedures

- Fast-running tap water and toilet flushes

- Sneezing, coughing, exhaling

- Droplet nuclei suspended in air due to evaporation of droplets


Sources of Transmission by Aerosols

•   Direct spray of large droplets onto conjunctiva/other mucous
     membranes via talking, coughing, sneezing, etc.  
•  Touching fomites (infected surfaces of inanimate objects)


Sources of Transmission by Droplets 

- temperature

- humidity

- radiation

- open/closed space

- safeguard present or absent


Factors Affecting Aerosol and Droplet Dispersion

Studying the generation and mobility of droplets and aerosols are of the utmost importance in order to develop adequate safeguards. The improvement of these safeguards is crucial, especially for healthcare workers who are exposed to viral particles via patients infected with COVID-19. 


Safeguards Against Transmission of Aerosols and Droplets 

This article argues that if COVID-19 relied on primarily airborne transmission, then its reproductive number (R0) should be considerably higher. 

A comparison to another respiratory virus that relies mainly on airborne transmission, the measles, shows that COVID's R0 value is much less than the measles, which had a value of 18. This might be partially due to the efficacy of existing safeguards.


Theoretical Consideration of COVID-19's Reproductive Number

The COVID-19 pandemic results in unprecedented containment and mitigation policies intended to slow the transmission of SARS-CoV-2. While these measures have been proven to be effective, the host-to-host transmission models for respiratory infectious diseases are predicated on a transmission model that doesn’t reflect transmission complexity. This study evaluates the understanding of turbulent gas clouds that can carry clusters of droplets and stay in the air longer than isolated droplets.

Turbulent Gas Clouds and Respiratory Pathogen Emissions

The authors of this study assessed the stability of SARS-CoV-2 and SARS-CoV-1 in aerosols and on various surfaces.

Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1

A study examining the evidence for Covid-19 transmission through aerosol concluded that the claim is supported through three reasonings:
- Cases of asymptomatic patients transmitting the virus to other people
- Some studies have recorded aerosolized SARS‐CoV‐2 particles that have stayed in the air for an extended period.
- Other viruses similar to Covid-19 have been recorded with aerosol transmission





Consideration of the Aerosol Transmission for COVID‐19 and Public Health

To understand the aerosol transmission route of SARS-CoV-2, this study aimed to:
-	Quantify the concentrations of airborne SARS-CoV-2 both inside hospitals and in outdoor areas where public gatherings are common
-	Evaluate the aerodynamic size distributions of SARS-CoV-2 aerosols that may facilitate its airborne transmission in different settings
-	Determine the dry deposition rate of the airborne SARS-CoV-2 in an ICU patient ward room

It concluded that:
- Room ventilation, open space, and proper disinfection of toilets can limit aerosol transmission of virus particles
- Crowds with asymptomatic carriers is a potential source of airborne SARS-CoV-2
- The virus aerosol deposition on PPE or floor surfaces and subsequent resuspension is a potential transmission pathway
- Rigorous sanitization protocols are critical in slowing the spread of the virions

Study on Aerodynamic Characteristics and RNA Concentration of SARS-CoV-2 Aerosols in Wuhan, China

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.



Smith College

University of South Florida

University of California, Davis

Likewise with droplets, aerosols are also “globs” of saliva, mucus and water, but are considerably smaller. Because of their size, they evaporate faster and travel farther, and usually remain airborne for a prolonged period of time. Aerosols usually have an aerodynamic diameter smaller than 5um.. They are more susceptible to being inhaled and reaching the alveolar region of the lungs, due to to their small size. There are also cases where droplets evaporate and eventually become aerosol particles, referred to as droplet nuclei. 

Aerosols

- Current lack of understanding of the prevalence and efficacy of antibodies in individuals exposed to both milder and more acute COVID
- If there is immunity from antibodies, the duration of immunity is unknown
- Subsequent inability to determine how much current herd immunity is in a population or is required to safely reopen economies 
- Present medical and public policy nuances and challenges associated with safe and effective development and distribution of a vaccine to populations

Key Public Health Considerations for COVID-19 Related Policy

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

When an infected person expels air (be it through coughing, sneezing, talking, etc), viral load travels in “globs” of various sizes. Some of these “globs” are too heavy to remain in the air, fall at a faster rate than they evaporate, and thus land on various surfaces. These particles are referred to as droplets, and are usually defined as having an aerodynamic diameter greater than 5μm.

Droplets

Jayaweera, M., Perera, H., Gunawardana, B., & Manatunge, J. (2020). Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy. Environmental Research, 188, 109819. Advance online publication.

doi: 10.1016/j.envres.2020.109819

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7293495/

Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy

Aerosol versus Droplet Debate

Face masks are thought to reduce or prevent transmission of the viral particles through aerosol transmission. The usage of face masks is a controversial topic whose efficacy is disputed by governments, researchers, and the public. Health officials in the early stages of the pandemic advised against the use of face masks for various reasons, although recent developments in the area of research may influence their perspectives.

Face Masks to Prevent the Spread of COVID-19

Altmann, Daniel M., Douek, Daniel C., Boyton, Rosemary J. What policy makers need to know about COVID-19 protective immunity. The Lancet, 395, 10236, 1527-1529 (2020). https://doi.org/10.1016/S0140-6736(20)30985-5 https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)30985-5/fulltext#articleInformation

What Policy Makers need to know about COVID-19 Protective Immunity 

References for Control Strategies of COVID-19 by Newspaper/Organization

- The authority of the government to mandate public health actions, including quarantine, isolation, immunization, contact tracing, property seizures, and environmental regulation are justified by three factors
- Factors include when the health of the population is threatened, when government action is more efficient or more likely to produce an effective intervention, and when the government has unique powers and expertise related to an essential aspect of public health
- The government’s role is to strike a balance between autonomy and paternalism to eliminate a threat to the public’s health

Justifications for Government Public Health Intervention

- Public Health plays a role scientific, ethical, legal, and social structures 
- A greater understanding and consensus on the meaning of public health could lead to “more efficient and effective public health interventions as well as increased public and political support for public health activities” 
- Many definitions classify public health as population health rather than individual 
- Definitions vary in who is viewed as responsible for public health, we as a society vs the government



Defining Public Health 

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

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

Implications of Asymptomatic COVID-19 Transmission

- 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

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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