Transmission of COVID-19

Last updated

Transmission of COVID-19
Other namesMode of spread of COVID-19
Covid-19 Aerosol.jpg
The respiratory route of spread of COVID-19, encompassing larger droplets and aerosols.
Specialty Infection prevention and control
Types Respiratory droplet, airborne transmission, fomites
PreventionFace coverings, quarantine, physical/social distancing, ventilation, hand washing, vaccination

The transmission of COVID-19 is the passing of coronavirus disease 2019 from person to person. The disease is mainly transmitted via the respiratory route when people inhale droplets and small airborne particles (that form an aerosol) that infected people exhale as they breathe, talk, cough, sneeze, or sing. [1] [2] [3] [4] Infected people are more likely to transmit COVID-19 when they are physically close. However, infection can occur over longer distances, particularly indoors. [1] [5]


Infectivity can occur 1-3 days before the onset of symptoms. [6] Infected persons can spread the disease even if they are pre-symptomatic or asymptomatic. [6] Most commonly, the peak viral load in upper respiratory tract samples occurs close to the time of symptom onset and declines after the first week after symptoms begin. [6] Current evidence suggests a duration of viral shedding and the period of infectiousness of up to 10 days following symptom onset for persons with mild to moderate COVID-19, and a up to 20 days for persons with severe COVID-19, including immunocompromised persons. [7] [6]

Infectious particles range in size from aerosols that remain suspended in the air for long periods of time to larger droplets that remain airborne or fall to the ground. [8] [9] [10] [11] Additionally, COVID-19 research has redefined the traditional understanding of how respiratory viruses are transmitted. [11] [12] The largest droplets of respiratory fluid do not travel far, and can be inhaled or land on mucous membranes on the eyes, nose, or mouth to infect. [10] Aerosols are highest in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close, [10] [11] [12] but airborne transmission can occur at longer distances, mainly in locations that are poorly ventilated; [10] in those conditions small particles can remain suspended in the air for minutes to hours. [10]

The number of people generally infected by one infected person varies; [13] as only 10 to 20% of people are responsible for the disease's spread. [14] It often spreads in clusters, where infections can be traced back to an index case or geographical location. [15] Often in these instances, superspreading events occur, where many people are infected by one person. [13]

A person can get COVID-19 indirectly by touching a contaminated surface or object before touching their own mouth, nose, or eyes, [6] [16] though strong evidence suggests this does not contribute substantially to new infections. [10] Although it is considered possible, there is no direct evidence of the virus being transmitted by skin to skin contact. [13] The virus is not known to spread through feces, urine, breast milk, food, wastewater, drinking water, or animal disease vectors (though some animals can contract the virus from humans). [16] [17] It very rarely transmits from mother to baby during pregnancy. [13]

Infectious period

After people are infected with COVID-19, they are able to transmit the disease to other people from one to three days before developing symptoms, known as presymptomatic transmission. [6] Contact tracing is used to find and contact people whom have been in contact with an infected individual in the 48 to 72 hours before they develop symptoms, or before their test date if asymptomatic. [6]

People are most infectious when they show symptoms—even if mild or non-specific—as the viral load is highest at this time. [6] [16]

Based on current evidence, adults with mild to moderate COVID-19 remain infectious (i.e., shed replication-competent SARS-CoV-2) for up to 10 days after symptoms begin. Adults with severe to critical COVID-19, or severe immune suppression (immunocompromised persons), may remain infectious (i.e., shed replication-competent SARS-CoV-2) for up to 20 days after symptoms begin. [18] [7]

Asymptomatic transmission

People who are asymptomatic are able to transmit the virus. [10] A December 2020 systematic review estimated that about 17% of COVID-19 infections were asymptomatic (95% confidence interval of 14% to 20%; the review found that "the transmission risk from asymptomatic cases appeared to be lower than that of symptomatic cases, but there was considerable uncertainty in the extent of this." [19] Persons with asymptomatic COVID-19 infection they can have the same viral load as symptomatic and presymptomatic cases, and are able to transmit the virus. [6] However, the infectious period of asymptomatic cases has been observed to be shorter with faster viral clearance. [6]

Dominant mode of transmission: airborne/aerosol

Our breath, shown here when speaking, forms a roughly cone-shaped plume of warm humid air, that breaks up into rolls. [20] The virus-containing droplets in the breath of an infected person, are carried out into the surroundings, by this plume (person speaking on right hand side of screen).

The dominant mode of transmission of the COVID-19 virus is exposure to respiratory droplets (small liquid particles) carrying infectious virus (i.e., airborne or aerosol transmission). [8] [21] [22] [23] [24] [25] [9] [26] Spread occurs when the particles are emitted from the mouth or nose of an infected person when they breathe, cough, sneeze, talk, or sing. [9] [27] [28] Human breath forms a roughly cone-shaped plume of air; in an infected person, the breath carries out the virus-containing droplets. [28] [20] So we expect the highest concentration of virus-containing droplets to be directly in front of an infected person, which suggests that the risk of transmission is greatest within three to six feet of the source of the infection. [8] [3] But breath contains many droplets that smaller than 100 micrometres in size, and these can stay suspended in the air for at least minutes and move across a room. [29] [30] [28] [31] [32] There is evidence that infectious SARS-CoV-2 survives in aerosols for a few hours. [33] There is substantial evidence for transmission events across a room (i.e., over distances larger than a metre or two) that is associated with being indoors, particularly in poorly ventilated spaces, although even indoor air drafts driven by air conditioning systems may contribute to the spread of respiratory sections. [5] [34] [35] This has led to statements that transmission occurs most easily in the "three C's": crowded places, close contact settings, and confined and enclosed spaces. [9]

Video explainer on reducing airborne transmission of COVID-19 indoors

This mode of transmission occurs via an infected person breathing out the virus, which is then carried by the air to a person nearby, or to someone across a room, who then breathes the virus in. Attempts to reduce airborne transmission act on one or more of these steps in transmission. [36] Masks or face coverings are worn to reduce the virus breathed out by an infected person (who may not know they are infected), as well as the virus breathed in by a susceptible person. Social distancing keeps people apart. To prevent virus building up in the air of a room occupied by one or more infected people, [36] ventilation is used to vent virus-laden air to the outside (where it will be diluted in the atmosphere) and replace it with virus-free air from the outside. Alternatively, the air may passed through filters to remove the virus-containing particles. A combination of shielding (protection from large droplet ejection) and air filtering, eliminating aerosols, ("Shield and sink" strategy) is particularly effective in reducing transfer of respiratory materials in indoor settings. [37]

Because physical intimacy and sex involve close contact, public health authorities have discouraged unvaccinated persons and persons with COVID-19 from engaging in kissing, casual sex, or other activities. [38]

The risk of transmission from all size droplets and aerosols is lower in indoor spaces with good ventilation. [39] The risk of outdoor transmission is low. [40] [41]

Transmission events occur in workplaces, schools, conferences, sporting venues, dormitories, prisons, shopping facilities, and ships, [42] as well as restaurants, [35] passenger vehicles, [43] religious buildings and choir practices, [44] and hospitals and other healthcare settings. [45] A superspreading event in a Skagit County, Washington, choral practice resulted in 32 to 52 of the 61 attendees infected. [46] [5]

An existing model of airborne transmission (the Wells-Riley model) was adapted to help understand why crowded and poorly ventilation spaces promote transmission, [5] with findings supported by aerodynamic analysis of droplet transfer in air-conditioned hospital rooms. [34] Airborne transmission also occurs in healthcare settings; long distance dispersal of virus particles has been detected in ventilation systems of a hospital. [45]

Some scientists criticized public health authorities in 2020 for being too slow to recognize airborne (aerosol) transmission of COVID-19, and to update their public health guidance accordingly. [47] [48] [49] By mid-2020, the public health authorities had all updated their guidance to reflect the importance of airborne transmission. [8] [50] [9]

Medical procedures designated as aerosol-generating procedures

There is concern that some medical procedures that affect the mouth and lungs can also generate aerosols, and that this may increase the infection risk. Some medical procedures have been designated as aerosol-generating procedures (AGPs), [9] [51] but this been done without measuring the aerosols these procedures produce. [52] The aerosols generated by some AGPs have been measured and found to be less than the aerosols produced by breathing [53] Less virus (strictly viral RNA) has been found in the air near intensive care unit (ICUs) with COVID-19 patients than near rooms with COVID-19 patients that are not ICUs. [54] Patients in ICUs are more likely to be subject to mechanically ventilation, an AGP. This suggests that in hospitals, areas near ICUs may actually pose less risk of infection via aerosols. This has led to calls to reconsider AGPs. [52] The WHO recommends the use of filtering facepiece respirators such as N95 or FFP2 masks in settings where aerosol-generating procedures are performed, [16] while the U.S. CDC and the European Centre for Disease Prevention and Control recommend these controls in all situations related to COVID-19 patient treatment (other than during crisis shortages). [55] [56] [57]

Rarer modes of transmission

Surface (fomite) transmission

Surfaces that are often touched such as door handles may transmit COVID-19, although is not thought to be the main way the virus spreads. Door Handle.JPG
Surfaces that are often touched such as door handles may transmit COVID-19, although is not thought to be the main way the virus spreads.

A person can get COVID-19 by touching a surface or object that has the virus on it (called a fomite), and then touching their own mouth, nose, or eyes, but it is not the main mode of transmission, and the risk of surface transmission is low. [23] [9] [13] [16] [18] [21] As of July 2020, "no specific reports which have directly demonstrated fomite transmission" although "People who come into contact with potentially infectious surfaces often also have close contact with the infectious person, making the distinction between respiratory droplet and fomite transmission difficult to discern." [16]

Each contact with a surface contaminated with SARS-CoV-2 has less than a 1 in 10,000 chance of causing an infection. [23] Various surface survival studies have found no detectable viable virus on porous surfaces within minutes to hours, but have found viable virus persisting on non-porous surfaces for days to weeks. [23] [16] However, surface-survival studies do not reflect real-world conditions, which are less favorable to the virus. [23] Ventilation and changes in environmental conditions can kill or degrade the virus. [16] [23] For example, temperature, humidity, and ultraviolet radiation (sunlight) all influence reductions in viral viability and infectiousness on surfaces. [8] Fomite transmission risk is also reduced because the virus does not transfer efficiently from the surface to the hands, and then from the hands to the mucous membranes (mouth, nose, and eye). [23]

The initial amount of virus on the surface (i.e., the viral load in respiratory droplets) also affects fomite transmission risk. [23] Hand washing and periodic surface cleaning impede indirect contact transmission through fomites. [9] [21] [23] Fomite transmission can be easily prevented with use of regular household cleaners or disinfection. [23] [9] [58] When surface survival data and factors affecting real-world transmission are considered, "the risk of fomite transmission after a person with COVID-19 has been in an indoor space is minor after 3 days (72 hours), regardless of when it was last cleaned." [23]

Animal vectors

Although the COVID-19 virus likely originated in bats, the pandemic is sustained through human-to-human spread, and the risk of animal-to-human spread of COVID-19 is low. [59] [60] COVID-19 infections in non-human animals have included companion animals (e.g., domestic cats, dogs, and ferrets), zoo and animal sanctuary residents (e.g., big cats, otters, and non-human primates); mink in mink farms in multiple countries; and wild white-tailed deer in numerous U.S. states. [59] Most animal infections came after the animals were in close contact with a human with COVID-19, such as an owners or caretaker. [59] Experimental research in laboratory settings also shows that other types of mammals (e.g., voles, rabbits, hamsters, pigs, macaques, baboons) can become infected. [59] By contrast, chickens and ducks do not seem to become infected with, or spread, the virus. [59] There is no evidence that the COVID-19 virus can spread to humans from the skin, fur, or hair of pets. [60] The U.S. CDC recommended that pet owners limit their pet's interactions with unvaccinated people outside their household; advises pet owners not to put face coverings on pets, as it could harm them; and states that pets should not be disinfected with cleaning products not approved for animal use. [60] If a pet becomes sick with COVID-19, the CDC recommends that owners "follow similar recommended precautions as for people caring for an infected person at home." [60]

People sick with COVID-19 should avoid contact with pets and other animals, in the same manner that people sick with COVID-19 should avoid contact with people. [60]

Vectors for which there is no evidence of COVID-19 transmission

Mother to child

The is no evidence for intrauterine transmission of COVID-19 from pregnant women to their fetuses. [16] Studies have not found any viable virus in breast milk. [16] Breast milk is unlikely to spread the COVID-19 virus to babies. [61] [62] Noting the benefits of breastfeeding, the WHO recommends that mothers with suspected or confirmed COVID-19 should be encouraged to initiate or continue to breastfeed, while taking proper infection prevention and control measures. [62] [16]

Food and water

No evidence suggests that handling food or consuming food is associated with transmission of COVID-19. [63] [64] The COVID-19 virus had poor survivability on surfaces; [63] less than 1 in 10,000 contacts with contaminated surfaces, including non-food-related surfaces, lead to infection. [23] As a result, the risk of spread from food products or packaging is very low. [64] Public health authorities recommend that people follow practice good hygiene by wash hands with soap and water before preparing and consuming food. [63] [64]

The COVID-19 virus has not been detected in drinking water. [65] Conventional water treatment (filtration and disinfection) inactivates or removes the virus. [65] COVID-19 virus RNA is found in untreated wastewater, [65] [17] [66] but there is no evidence of COVID-19 transmission through exposure to untreated wastewater or sewerage systems. [65] There is also no evidence that COVID-19 transmission to humans occurs through water in swimming pools, hot tubs, or spas. [65]


While SARS-CoV-2 RNA has been detected in the urine and feces of some persons infected with COVID-19, there is no evidence of COVID-19 transmission through feces or urine. [16] [65] COVID-19 is not an insect-borne disease; there is also no evidence that mosquito are a vector for COVID-19. [67] COVID‑19 is not a sexually transmitted infection; while the virus has been found in the semen of people who have COVID-19, there is no evidence that the virus spreads through semen or vaginal fluid. [38]

Clusters and other patterns

Many people do not transmit the virus, but some transmit to many people, and the virus is considered to be "overdispersed" - the transmission rate has high heterogeneity. [13] [68] "Super-spreading events" occur from this minority of infected people, usually in high risk venues including restaurants, nightclubs, places of worship, and they generally occur indoors. [13] It often spreads in these clusters, where infections can be traced back to an index case or geographical location. [15] These generally occur usually indoors, where groups of people remain in poor ventilation for longer periods. [13] It transmits via aerosols particularly in these crowded and confined indoor spaces, which are particularly effective for transmitting the virus, [9] such as restaurants, nightclubs or choirs. [69] Another important site for transmission is between members of the same household. [13]

COVID-19 is more infectious than influenza, but less so than measles. [21] Estimates of the number of people infected by one person with COVID-19the basic reproduction number (R0have varied. In November 2020, a systematic review estimated R0 of the original Wuhan strain to be approximately 2.87 (95% CI, 2.393.44). [70] The R0 of the Delta variant, which became the dominant variant of COVID-19 in 2021, is substantially higher. Among five studies catalogued in October 2021, Delta's mean estimate R0 was 5.08. [71]

Effect of vaccination

The Pfizer-BioNTech, Moderna, and Janssen COVID-19 vaccines provide effective protection against COVID-19, including against severe disease, hospitalization, and death, and "a growing body of evidence suggests that COVID-19 vaccines also reduce asymptomatic infection and transmission" as chains of transmission are interrupted by vaccines. [72] While fully vaccinated people can still become infected and potentially transmit the virus to others (particularly in areas of widespread community transmission), they do so at a much lower rate than unvaccinated people. [72] The primary cause of continued spread of COVID-19 is transmission between unvaccinated people. [72]

Related Research Articles

Epidemic Rapid spread of disease affecting a large number of people in a short time

An epidemic is the rapid spread of disease to a large number of people in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic.

Severe acute respiratory syndrome Disease caused by severe acute respiratory syndrome coronavirus

Severe acute respiratory syndrome (SARS) is a viral respiratory disease of zoonotic origin caused by severe acute respiratory syndrome coronavirus, the first identified strain of the SARS coronavirus species severe acute respiratory syndrome–related coronavirus (SARSr-CoV). The syndrome caused the 2002–2004 SARS outbreak. Around late 2017, Chinese scientists traced the virus through the intermediary of Asian palm civets to cave-dwelling horseshoe bats in Xiyang Yi Ethnic Township, Yunnan.

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected. The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:

Surgical mask Mouth and nose cover against bacterial aerosols

A surgical mask, also known as a medical face mask, is a personal protective equipment worn by health professionals during medical procedures. When worn correctly, it prevents airborne transmission of infections between patients and/or treating personnel by blocking the movement of pathogens shed in respiratory droplets and aerosols from the wearer's mouth and nose.

Natural reservoir Living host, such as an animal or a plant, inside of which an infectious pathogen naturally lives and reproduces

In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and reproduces, or upon which the pathogen primarily depends for its survival. A reservoir is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often without causing disease for the reservoir itself. By some definitions a reservoir may also be an environment external to an organism, such as a volume of contaminated air or water.

Contact tracing Finding and identifying people in contact with someone with an infectious disease

In public health, contact tracing is the process of identifying persons who may have come into contact with an infected person ("contacts") and subsequent collection of further information about these contacts. By tracing the contacts of infected individuals, testing them for infection, isolating or treating the infected, and tracing their contacts, public health aims to reduce infections in the population. Diseases for which contact tracing is commonly performed include tuberculosis, vaccine-preventable infections like measles, sexually transmitted infections, blood-borne infections, Ebola, some serious bacterial infections, and novel virus infections. The goals of contact tracing are:

<i>Human coronavirus NL63</i> Species of virus

Human coronavirus NL63 (HCoV-NL63) is a species of coronavirus, specifically a Setracovirus from among the Alphacoronavirus genus. It was identified in late 2004 in a seven-month-old child with bronchiolitis in the Netherlands. The virus is an enveloped, positive-sense, single-stranded RNA virus which enters its host cell by binding to ACE2. Infection with the virus has been confirmed worldwide, and has an association with many common symptoms and diseases. Associated diseases include mild to moderate upper respiratory tract infections, severe lower respiratory tract infection, croup and bronchiolitis.

Isolation (health care) Measure taken to prevent contagious diseases from being spread

In health care facilities, isolation represents one of several measures that can be taken to implement in infection control: the prevention of communicable diseases from being transmitted from a patient to other patients, health care workers, and visitors, or from outsiders to a particular patient. Various forms of isolation exist, in some of which contact procedures are modified, and others in which the patient is kept away from all other people. In a system devised, and periodically revised, by the U.S. Centers for Disease Control and Prevention (CDC), various levels of patient isolation comprise application of one or more formally described "precaution".

Airborne transmission Disease transmission by airborne particles

Airborne or aerosol transmission is transmission of an infectious disease through small particles suspended in the air. Infectious diseases capable of airborne transmission include many of considerable importance both in human and veterinary medicine. The relevant infectious agent may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, flushing toilets, or any activities which generate aerosol particles or droplets.

A fomite or fomes is any inanimate object that, when contaminated with or exposed to infectious agents, can transfer disease to a new host. In the 21st century, the role of fomites in disease transfer is higher than ever in human history because of indoor lifestyle.

Wells curve Science of medicine

The Wells curve is a diagram, developed by W. F. Wells in 1934, which describes what is expected to happen to small droplets once they have been exhaled into air. Coughing, sneezing, and other violent exhalations produce high numbers of respiratory droplets derived from saliva and/or respiratory mucus, with sizes ranging from about 1 µm to 2 mm. Wells' insight was that that such droplets would have two distinct fates, depending on their sizes. The interplay of gravity and evaporation means that droplets larger than a humidity-determined threshold size would fall to the ground due to gravity, while droplets smaller than this size would quickly evaporate, leaving a dry residue that drifts in the air. Since droplets from an infected person may contain infectious bacteria or viruses, these processes influence transmission of respiratory diseases.

Respiratory droplet Type of particle formed by breathing

A respiratory droplet is a small aqueous droplet produced by exhalation, consisting of saliva or mucus and other matter derived from respiratory tract surfaces. Respiratory droplets are produced naturally as a result of breathing, speaking, sneezing, coughing, or vomiting, so they are always present in our breath, but speaking and coughing increases their number. Droplet sizes range from < 1 µm to 1000 µm, and in typical breath there are around 100 droplets per litre of breath. So for a breathing rate of 10 litres per minute this means roughly 1000 droplets per minute, the vast majority of which are a few micrometres across or smaller. As these droplets are suspended in air, they are all by definition aerosols. However, large droplets rapidly fall to the ground or another surface and so are only briefly suspended, while droplets much smaller than 100 µm fall only slowly and so form aerosols with lifetimes of minutes or more, or at intermediate size, may initially travel like aerosols but at a distance fall to the ground like droplets. As the droplets are so small they dry rapidly once in the surrounding air, shrink, and therefore remain suspended for a longer time.

Severe acute respiratory syndrome coronavirus 2 Virus that causes COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2), also known as the coronavirus, is the virus that causes COVID-19, the respiratory illness responsible for the ongoing COVID-19 pandemic. The virus was previously referred to by its provisional name, 2019 novel coronavirus (2019-nCoV), and has also been called human coronavirus 2019. First identified in the city of Wuhan, Hubei, China, the World Health Organization declared the outbreak a Public Health Emergency of International Concern on 30 January 2020, and a pandemic on 11 March 2020. SARS‑CoV‑2 is a positive-sense single-stranded RNA virus that is contagious in humans. As described by the US National Institutes of Health, it is the successor to SARS-CoV-1, the virus that caused the 2002–2004 SARS outbreak.

Dental aerosol Hazardous biological compound

A dental aerosol is an aerosol that is produced from dental instrument, dental handpieces, three-way syringes, and other high-speed instruments. These aerosols may remain suspended in the clinical environment. Dental aerosols can pose risks to the clinician, staff, and other patients. The heavier particles contained within the aerosols are likely to remain suspended in the air for relatively short period and settle quickly onto surfaces, however, the lighter particles may remain suspended for longer periods and may travel some distance from the source. These smaller particles are capable of becoming deposited in the lungs when inhaled and provide a route of diseases transmission.

COVID-19 Contagious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)

Coronavirus disease 2019 (COVID-19) is a contagious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019. The disease has since spread worldwide, leading to an ongoing pandemic.

Cloth face mask mask made of common textiles worn over the mouth and nose

A cloth face mask is a mask made of common textiles, usually cotton, worn over the mouth and nose. When more effective masks are not available, and when physical distancing is impossible, cloth face masks are recommended by public health agencies for disease "source control" in epidemic situations to protect others from virus laden droplets in infected mask wearers' breath, coughs, and sneezes. Because they are less effective than N95 masks, surgical masks, or physical distancing in protecting the wearer against viruses, they are not considered to be personal protective equipment by public health agencies. They are used by the general public in household and community settings as protection against both infectious diseases and particulate air pollution.

Allison McGeer is a Canadian infectious disease specialist in the Sinai Health System, a Professor at the Dalla Lana School of Public Health and a Senior Clinician Scientist at the Lunenfeld-Tanenbaum Research Institute. McGeer has led investigations into the severe acute respiratory syndrome outbreak in Toronto and worked alongside Donald Low. During the COVID-19 pandemic, McGeer has studied how SARS-CoV-2 survives in the air.

Human-to-human transmission (HHT) is a particularly problematic epidemiologic vector, especially in case the disease is borne by individuals known as superspreaders. In these cases, the basic reproduction number of the virus, which is the average number of additional people that a single case will infect without any preventative measures, can be as high as 3.9. Interhuman transmission is a synonym for HHT.

Source control (respiratory disease) Strategy for reducing disease transmission

Source control is a strategy for reducing disease transmission by blocking respiratory secretions produced through speaking, coughing, sneezing or singing. Surgical masks are commonly used for this purpose, with cloth face masks recommended for use by the public only in epidemic situations when there are shortages of surgical masks. In addition, respiratory etiquette such as covering the mouth and nose with a tissue when coughing can be considered source control. In diseases transmitted by droplets or aerosols, understanding air flow, particle and aerosol transport may lead to rational infrastructural source control measures that minimize exposure of susceptible persons.

The Wells-Riley model is a simple model of the airborne transmission of infectious diseases, developed by William F. Wells and Richard L. Riley for tuberculosis and measles.


    1. 1 2 Wang CC, Prather KA, Sznitman J, Jimenez JL, Lakdawala SS, Tufekci Z, Marr LC (August 2021). "Airborne transmission of respiratory viruses". Science. 373 (6558). Bibcode:2021Sci...373.....W. doi: 10.1126/science.abd9149 . PMID   34446582.
    2. Greenhalgh T, Jimenez JL, Prather KA, Tufekci Z, Fisman D, Schooley R (May 2021). "Ten scientific reasons in support of airborne transmission of SARS-CoV-2". Lancet. 397 (10285): 1603–1605. doi:10.1016/s0140-6736(21)00869-2. PMC   8049599 . PMID   33865497.
    3. 1 2 Bourouiba L (13 July 2021). "Fluid Dynamics of Respiratory Infectious Diseases". Annual Review of Biomedical Engineering. 23 (1): 547–577. doi:10.1146/annurev-bioeng-111820-025044. hdl: 1721.1/131115 . PMID   34255991. S2CID   235823756 . Retrieved 7 September 2021.
    4. Stadnytskyi, Valentyn; Bax, Christina E.; Bax, Adriaan; Anfinrud, Philip (2 June 2020). "The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission". Proceedings of the National Academy of Sciences. 117 (22): 11875–11877. doi:10.1073/pnas.2006874117. PMC   7275719 . PMID   32404416.
    5. 1 2 3 4 Miller SL, Nazaroff WW, Jimenez JL, Boerstra A, Buonanno G, Dancer SJ, et al. (March 2021). "Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event". Indoor Air. 31 (2): 314–323. doi:10.1111/ina.12751. PMC   7537089 . PMID   32979298.
    6. 1 2 3 4 5 6 7 8 9 10 Communicable Diseases Network Australia. "Coronavirus Disease 2019 (COVID-19): CDNA National Guidelines for Public Health Units". 5.1. Communicable Diseases Network Australia/Australian Government Department of Health.
    7. 1 2 "Clinical Questions about COVID-19: Questions and Answers". Centers for Disease Control and Prevention. 4 March 2021.
    8. 1 2 3 4 5 "Scientific Brief: SARS-CoV-2 Transmission". Centers for Disease Control and Prevention. 7 May 2021. Retrieved 8 May 2021.
    9. 1 2 3 4 5 6 7 8 9 10 "Coronavirus disease (COVID-19): How is it transmitted?". World Health Organization. 30 April 2021.
    10. 1 2 3 4 5 6 7   "COVID-19: epidemiology, virology and clinical features". GOV.UK. Retrieved 18 October 2020.
       Communicable Diseases Network Australia. "Coronavirus Disease 2019 (COVID-19) - CDNA Guidelines for Public Health Units". Version 4.4. Australian Government Department of Health. Retrieved 17 May 2021.
       Public Health Agency of Canada (3 November 2020). "COVID-19: Main modes of transmission". aem. Retrieved 18 May 2021.
        "Transmission of COVID-19". European Centre for Disease Prevention and Control. Retrieved 18 May 2021.
       Meyerowitz EA, Richterman A, Gandhi RT, Sax PE (January 2021). "Transmission of SARS-CoV-2: A Review of Viral, Host, and Environmental Factors". Annals of Internal Medicine. 174 (1): 69–79. doi:10.7326/M20-5008. PMC   7505025 . PMID   32941052.
    11. 1 2 3 Tang JW, Marr LC, Li Y, Dancer SJ (April 2021). "Covid-19 has redefined airborne transmission". BMJ. 373: n913. doi: 10.1136/bmj.n913 . PMID   33853842.
    12. 1 2 Morawska L, Allen J, Bahnfleth W, Bluyssen PM, Boerstra A, Buonanno G, et al. (May 2021). "A paradigm shift to combat indoor respiratory infection" (PDF). Science. 372 (6543): 689–691. Bibcode:2021Sci...372..689M. doi:10.1126/science.abg2025. PMID   33986171. S2CID   234487289.
    13. 1 2 3 4 5 6 7 8 9 Meyerowitz EA, Richterman A, Gandhi RT, Sax PE (January 2021). "Transmission of SARS-CoV-2: A Review of Viral, Host, and Environmental Factors". Annals of Internal Medicine. 174 (1): 69–79. doi:10.7326/M20-5008. PMC   7505025 . PMID   32941052.
    14. Lessler J, Grantz K. "Overdispersion of COVID-19". Johns Hopkins Bloomberg School of Public Health. Retrieved 11 May 2021.
    15. 1 2 Liu T, Gong D, Xiao J, Hu J, He G, Rong Z, Ma W (October 2020). "Cluster infections play important roles in the rapid evolution of COVID-19 transmission: A systematic review". International Journal of Infectious Diseases. 99: 374–380. doi:10.1016/j.ijid.2020.07.073. PMC   7405860 . PMID   32768702.
    16. 1 2 3 4 5 6 7 8 9 10 11 12 "Transmission of SARS-CoV-2: implications for infection prevention precautions" (PDF). World Health Organization. 9 July 2020.
    17. 1 2 "Water, sanitation, hygiene, and waste management for SARS-CoV-2, the virus that causes COVID-19" (PDF). . 29 July 2020. Retrieved 14 October 2020.
    18. 1 2 "Q & A on COVID-19: Basic facts". European Centre for Disease Prevention and Control. 21 September 2021.
    19. Oyungerel Byambasuren, Magnolia Cardona, Katy Bell, Justin Clark, Mary-Louise McLaws, Paul Glasziou (December 2020). "Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: Systematic review and meta-analysis". Official Journal of the Association of Medical Microbiology and Infectious Disease Canada. 5 (4): 223–234. doi:10.3138/jammi-2020-0030. S2CID   234686396.CS1 maint: multiple names: authors list (link)
    20. 1 2 Abkarian M, Mendez S, Xue N, Yang F, Stone HA (October 2020). "Speech can produce jet-like transport relevant to asymptomatic spreading of virus". Proceedings of the National Academy of Sciences of the United States of America. 117 (41): 25237–25245. arXiv: 2006.10671 . Bibcode:2020PNAS..11725237A. doi:10.1073/pnas.2012156117. PMC   7568291 . PMID   32978297.
    21. 1 2 3 4 "How COVID-19 Spreads". Centers for Disease Control and Prevention. 14 July 2021.
    22. "COVID-19 Frequently Asked Questions". Centers for Disease Control and Prevention. 13 September 2021.
    23. 1 2 3 4 5 6 7 8 9 10 11 12 "Science Brief: SARS-CoV-2 and Surface (Fomite) Transmission for Indoor Community Environments". Centers for Disease Control and Prevention. 5 April 2021.
    24. Samet JM, Prather K, Benjamin G, Lakdawala S, Lowe JM, Reingold A, et al. (January 2021). "Airborne Transmission of SARS-CoV-2: What We Know". Clinical Infectious Diseases: ciab039. doi:10.1093/cid/ciab039. PMC   7929061 . PMID   33458756.
    25. Greenhalgh T, Jimenez JL, Prather KA, Tufekci Z, Fisman D, Schooley R (May 2021). "Ten scientific reasons in support of airborne transmission of SARS-CoV-2". Lancet. 397 (10285): 1603–1605. doi:10.1016/s0140-6736(21)00869-2. PMC   8049599 . PMID   33865497.
    26. "COVID-19: epidemiology, virology and clinical features". UK Health Security Agency. 6 October 2021.
    27. Santarpia JL, Herrera VL, Rivera DN, Ratnesar-Shumate S, Reid SP, Ackerman DN, et al. (August 2021). "The size and culturability of patient-generated SARS-CoV-2 aerosol". Journal of Exposure Science & Environmental Epidemiology: 1–6. doi:10.1038/s41370-021-00376-8. PMC   8372686 . PMID   34408261.
    28. 1 2 3 Bourouiba L (5 January 2021). "The Fluid Dynamics of Disease Transmission". Annual Review of Fluid Mechanics. 53 (1): 473–508. Bibcode:2021AnRFM..5360220B. doi: 10.1146/annurev-fluid-060220-113712 . ISSN   0066-4189. S2CID   225114407.
    29. de Oliveira PM, Mesquita LC, Gkantonas S, Giusti A, Mastorakos E (January 2021). "Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission". Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 477 (2245): 20200584. Bibcode:2021RSPSA.47700584D. doi: 10.1098/rspa.2020.0584 . PMC   7897643 . PMID   33633490.
    30. Lednicky JA, Lauzardo M, Fan ZH, Jutla A, Tilly TB, Gangwar M, et al. (November 2020). "Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients". International Journal of Infectious Diseases. 100: 476–482. doi:10.1016/j.ijid.2020.09.025. PMC   7493737 . PMID   32949774.
    31. Balachandar S, Zaleski S, Soldati A, Ahmadi G, Bourouiba L (2020). "Host-to-host airborne transmission as a multiphase flow problem for science-based social distance guidelines". International Journal of Multiphase Flow. 132: 103439. arXiv: 2008.06113 . doi:10.1016/j.ijmultiphaseflow.2020.103439. PMC   7471834 .
    32. Netz RR (August 2020). "Mechanisms of Airborne Infection via Evaporating and Sedimenting Droplets Produced by Speaking". The Journal of Physical Chemistry B. 124 (33): 7093–7101. doi:10.1021/acs.jpcb.0c05229. PMC   7409921 . PMID   32668904.
    33. van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, et al. (April 2020). "Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1". The New England Journal of Medicine. 382 (16): 1564–1567. doi: 10.1056/NEJMc2004973 . PMC   7121658 . PMID   32182409.
    34. 1 2 Hunziker, Patrick (1 October 2021). "Minimising exposure to respiratory droplets, 'jet riders' and aerosols in air-conditioned hospital rooms by a 'Shield-and-Sink' strategy". BMJ Open. 11 (10): e047772. doi:10.1136/bmjopen-2020-047772. ISSN   2044-6055. PMC   8520596 . PMID   34642190.
    35. 1 2 Li Y, Qian H, Hang J, Chen X, Cheng P, Ling H, et al. (June 2021). "Probable airborne transmission of SARS-CoV-2 in a poorly ventilated restaurant". Building and Environment. 196: 107788. doi:10.1016/j.buildenv.2021.107788. PMC   7954773 . PMID   33746341.
    36. 1 2 Prather KA, Wang CC, Schooley RT (June 2020). "Reducing transmission of SARS-CoV-2". Science. 368 (6498): 1422–1424. Bibcode:2020Sci...368.1422P. doi: 10.1126/science.abc6197 . PMID   32461212.
    37. Hunziker, Patrick (1 October 2021). "Minimising exposure to respiratory droplets, 'jet riders' and aerosols in air-conditioned hospital rooms by a 'Shield-and-Sink' strategy". BMJ Open. 11 (10): e047772. doi:10.1136/bmjopen-2020-047772. ISSN   2044-6055. PMC   8520596 . PMID   34642190.
    38. 1 2 "Safer Sex and COVID-19" (PDF). New York City Department of Health. 18 June 2021.
    39. The Lancet Respiratory Medicine Editors (December 2020). "COVID-19 transmission-up in the air". The Lancet. Respiratory Medicine. 8 (12): 1159. doi:10.1016/s2213-2600(20)30514-2. PMC   7598535 . PMID   33129420.
    40. Tommaso Celeste Bulfone, Mohsen Malekinejad, George W Rutherford, Nooshin Razani (15 February 2021). "Outdoor Transmission of SARS-CoV-2 and Other Respiratory Viruses: A Systematic Review". Journal of Infectious Diseases. 223 (4): 550–561. doi:10.1093/infdis/jiaa742. PMC   7798940 . PMID   33249484.CS1 maint: multiple names: authors list (link)
    41. "Participate in Outdoor and Indoor Activities". U.S. Centers for Disease Control and Prevention. 19 August 2021.
    42. Leclerc QJ, Fuller NM, Knight LE, Funk S, Knight GM (5 June 2020). "What settings have been linked to SARS-CoV-2 transmission clusters?". Wellcome Open Research. 5: 83. doi: 10.12688/wellcomeopenres.15889.2 . PMC   7327724 . PMID   32656368.
    43. Varghese Mathai, Asimanshu Das, Jeffrey A. Bailey & Kenneth Breuer (1 January 2021). "Airflows inside passenger cars and implications for airborne disease transmission". Science Advances. 7 (1). doi:10.1126/sciadv.abe0166. PMC   7775778 . PMID   33277325.CS1 maint: uses authors parameter (link)
    44. Katelaris AL, Wells J, Clark P, Norton S, Rockett R, Arnott A, et al. (June 2021). "Epidemiologic Evidence for Airborne Transmission of SARS-CoV-2 during Church Singing, Australia, 2020". Emerging Infectious Diseases. 27 (6): 1677–1680. doi:10.3201/eid2706.210465. PMC   8153858 . PMID   33818372.
    45. 1 2 Nissen K, Krambrich J, Akaberi D, Hoffman T, Ling J, Lundkvist Å, et al. (November 2020). "Long-distance airborne dispersal of SARS-CoV-2 in COVID-19 wards". Scientific Reports. 10 (1): 19589. Bibcode:2020NatSR..1019589N. doi:10.1038/s41598-020-76442-2. PMC   7659316 . PMID   33177563.
    46. Hamner L, Dubbel P, Capron I, Ross A, Jordan A, Lee J, et al. (May 2020). "High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice - Skagit County, Washington, March 2020". MMWR. Morbidity and Mortality Weekly Report. 69 (19): 606–610. doi: 10.15585/mmwr.mm6919e6 . PMID   32407303.
    47. Lewis D (July 2020). "Mounting evidence suggests coronavirus is airborne - but health advice has not caught up". Nature. 583 (7817): 510–513. Bibcode:2020Natur.583..510L. doi: 10.1038/d41586-020-02058-1 . PMID   32647382. S2CID   220470431.
    48. Zhang R, Li Y, Zhang AL, Wang Y, Molina MJ (June 2020). "Identifying airborne transmission as the dominant route for the spread of COVID-19". Proceedings of the National Academy of Sciences of the United States of America. 117 (26): 14857–14863. doi:10.1073/pnas.2009637117. PMC   7334447 . PMID   32527856.
    49. Tanne JH (September 2020). "Covid-19: CDC publishes then withdraws information on aerosol transmission". BMJ. 370: m3739. doi: 10.1136/bmj.m3739 . PMID   32973037. S2CID   221881893.
    50. "COVID-19: Main modes of transmission". Public Health Agency of Canada. 3 November 2020. Retrieved 25 November 2020.
    51. Tran K, Cimon K, Severn M, Pessoa-Silva CL, Conly J (2012). "Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: a systematic review". PLOS ONE. 7 (4): e35797. Bibcode:2012PLoSO...735797T. doi: 10.1371/journal.pone.0035797 . PMC   3338532 . PMID   22563403.
    52. 1 2 Hamilton F, Arnold D, Bzdek BR, Dodd J, Reid J, Maskell N (July 2021). "Aerosol generating procedures: are they of relevance for transmission of SARS-CoV-2?". The Lancet. Respiratory Medicine. 9 (7): 687–689. doi:10.1016/S2213-2600(21)00216-2. PMC   8102043 . PMID   33965002.
    53. Wilson NM, Marks GB, Eckhardt A, Clarke AM, Young FP, Garden FL, et al. (November 2021). "The effect of respiratory activity, non-invasive respiratory support and facemasks on aerosol generation and its relevance to COVID-19". Anaesthesia. 76 (11): 1465–1474. doi: 10.1111/anae.15475 . PMC   8250912 . PMID   33784793.
    54. Grimalt JO, Vílchez H, Fraile-Ribot PA, Marco E, Campins A, Orfila J, et al. (September 2021). "Spread of SARS-CoV-2 in hospital areas". Environmental Research. 204 (Pt B): 112074. doi:10.1016/j.envres.2021.112074. PMC   8450143 . PMID   34547251.
    55. "Infection prevention and control and preparedness for COVID-19 in healthcare settings - fifth update" (PDF).
    56. "Respiratory Protection During Outbreaks: Respirators versus Surgical Masks | | Blogs | CDC" . Retrieved 25 November 2020.
    57. CDC (11 February 2020). "Coronavirus Disease 2019 (COVID-19)". Centers for Disease Control and Prevention. Retrieved 29 November 2020.
    58. "COVID-19: Cleaning And Disinfecting Your Home". . 27 May 2020. Retrieved 7 October 2020.
    59. 1 2 3 4 5 "COVID-19 and Animals". 6 October 2021.
    60. 1 2 3 4 5 "COVID-19: If You Have Pets". . 29 June 2021.
    61. "Breastfeeding and Caring for Newborns if You Have COVID-19". 18 August 2021.
    62. 1 2 "Breastfeeding and COVID-19" (PDF). . World Health Organization. 23 June 2020. Archived from the original on 23 June 2020. Retrieved 18 September 2020.
    63. 1 2 3 "Questions and answers on COVID-19: Various". European Centre for Disease Prevention and Control. 8 September 2021.
    64. 1 2 3 "Food Safety and Coronavirus Disease 2019 (COVID-19)". U.S. Centers for Disease Control and Prevention. 22 June 2020.
    65. 1 2 3 4 5 6 "Water and COVID-19 FAQs: Information about Drinking Water, Treated Recreational Water, and Wastewater". U.S. Centers for Disease Control and Prevention. 23 April 2020.
    66. Corpuz MV, Buonerba A, Vigliotta G, Zarra T, Ballesteros F, Campiglia P, et al. (November 2020). "Viruses in wastewater: occurrence, abundance and detection methods". The Science of the Total Environment. 745: 140910. Bibcode:2020ScTEn.745n0910C. doi: 10.1016/j.scitotenv.2020.140910 . PMC   7368910 . PMID   32758747.
    67. Yan-Jang S. Huang, Dana L. Vanlandingham, Ashley N. Bilyeu, Haelea M. Sharp, Susan M. Hettenbach & Stephen Higgs (17 July 2020). "SARS-CoV-2 failure to infect or replicate in mosquitoes: an extreme challenge". Scientific Reports.CS1 maint: uses authors parameter (link)
    68. Endo A, Abbott S, Kucharski AJ, Funk S (2020). "Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China". Wellcome Open Research. 5: 67. doi:10.12688/wellcomeopenres.15842.3. PMC   7338915 . PMID   32685698.
    69. Kohanski MA, Lo LJ, Waring MS (October 2020). "Review of indoor aerosol generation, transport, and control in the context of COVID-19". International Forum of Allergy & Rhinology . 10 (10): 1173–1179. doi:10.1002/alr.22661. PMC   7405119 . PMID   32652898.
    70. Billah MA, Miah MM, Khan MN (11 November 2020). "Reproductive number of coronavirus: A systematic review and meta-analysis based on global level evidence". PLOS ONE. 15 (11): e0242128. Bibcode:2020PLoSO..1542128B. doi: 10.1371/journal.pone.0242128 . PMC   7657547 . PMID   33175914.
    71. Ying Liu & Joacim Rocklöv (October 2021). "The reproductive number of the Delta variant of SARS-CoV-2 is far higher compared to the ancestral SARS-CoV-2 virus". Journal of Travel Medicine. 28 (7). doi:10.1093/jtm/taab124. PMC   8436367 . PMID   34369565.CS1 maint: uses authors parameter (link)
    72. 1 2 3 "Science Brief: COVID-19 Vaccines and Vaccination". U.S. Centers for Disease Control and Prevention. 15 September 2021.