Transmission of COVID-19

The transmission of COVID-19 is the

Infectious particles range in size from aerosols that remain suspended in the air for long periods of time to larger droplets that remain airborne briefly or fall to the ground.^{[10][11][12][13]} Additionally, COVID-19 research has redefined the traditional understanding of how respiratory viruses are transmitted.^[13][14] The largest droplets of respiratory fluid do not travel far, but can be inhaled or land on mucous membranes on the eyes, nose, or mouth to infect.^[12] Aerosols are highest in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close,^[12][13][14] but airborne transmission can occur at longer distances, mainly in locations that are poorly ventilated;^[12] in those conditions small particles can remain suspended in the air for minutes to hours.^[12][15]

The number of people generally infected by one infected person varies,^[16] but it is estimated that the R₀ ("R nought" or "R zero") number is around 2.5.^[17] The disease often spreads in clusters, where infections can be traced back to an index case or geographical location.^[18] Often in these instances, superspreading events occur, where many people are infected by one person.^[16]

A person can get COVID-19

Infectious period

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After people are infected with COVID-19, they are able to transmit the disease to other people beginning as early as four to five days before developing symptoms, known as presymptomatic transmission.[8] To reduce such transmission, contact tracing is used to find and alert people who have been in contact with an infected individual in the 48 to 72 hours before they develop symptoms, or before that individual's test date if asymptomatic.^[8] Initial reports suggested that this early transmission was restricted to the two-to-three day time window,^[23] but an author correction later acknowledged that transmission could begin four to five days before symptom onset.^[7]

People are most infectious shortly before and after their symptoms begin^[7]—even if mild or non-specific—as the viral load peaks at this time.^[8][19]

Based on current evidence, adults with mild to moderate COVID-19 remain infectious (i.e., shed replication-competent SARS-CoV-2) for up to ten days after symptoms begin, although there are few transmission events are observed after five days.^[7] 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.^[24][9]

Patients who are tested positive to the virus again after recovery, in case they weren't being reinfected, is found to be not transmitting the virus to others.^[25]

Nearly a third of people with COVID-19 remain contagious five days after the onset of symptoms or a positive test. This is reduced to 7% for those who test negative twice with rapid tests on days 5 and 6. Without testing, 5% are contagious on day 10.^[26][27]

Asymptomatic transmission

People who are asymptomatic do not show symptoms but still are able to transmit the virus.^[12] At least a third of the people who are infected with the virus do not develop noticeable symptoms at any point in time.^[28][29][30] Asymptomatic carriers tend not to get tested.^[30][31][32]

Persons with asymptomatic COVID-19 infection can have the same viral load as symptomatic and presymptomatic cases, and are able to transmit the virus.^[8] However, the infectious period of asymptomatic cases has been observed to be shorter with faster viral clearance.^[8]

Dominant mode of transmission: airborne/aerosol

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).^{[10][34][35][36][37][2][11][38]} Spread occurs when the particles are emitted from the mouth or nose of an infected person when they breathe, cough, sneeze, talk, or sing.^[11][39][40] Human breath forms a roughly cone-shaped plume of air; in an infected person, the breath carries out the virus-containing droplets.^[40][33] 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.^[10][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.^{[41][42][40][43][44]} There is evidence that infectious SARS-CoV-2 survives in aerosols for a few hours.^[45] 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][46][47] 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.^[11]

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.^[48] 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,^[48] 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.^[49]

The sneeze resembles a free turbulent jet. The turbulent multiphase cloud contributes critically to increasing the range of the pathogen-bearing drops originating in human coughs and sneezes.^[50]  The jet's reach is nearly 22 ft in 18.5 seconds and 25 ft in 22 seconds.^[51] The shape of the expelled particles is conical, with a spreading angle of 23 degrees.^[51][52] The trajectory of the turbulent jet is inclined due to the inclination angle of the nose.^[51] Smaller droplets travel a considerable distance as freely suspended tracers and may still get reflected and follow the turbulent cloud.^[51] Droplets with a diameter less than 50 μm remain suspended in the cloud for an extended period of time, which allows the cloud to reach heights of 4 to 6 meters, where ventilation systems can be contaminated.^[50]

Because physical intimacy and sex involve close contact, in October 2021, New York City Department of Health discouraged unvaccinated persons, immunocompromised people, people over 65, persons with COVID-19, people with a health condition that increases the risk of severe COVID-19, and people who live with someone from one of these groups from engaging in kissing, casual sex, or other activities, and recommended wearing face mask during sex.^[53]

The risk of transmission from all size droplets and aerosols is lower in indoor spaces with good ventilation.^[54] The risk of outdoor transmission is low.^[55][56]

Transmission events occur in workplaces, schools, conferences, sporting venues, dormitories, prisons, shopping facilities, and ships,^[57] as well as restaurants,^[47] passenger vehicles,^[58] religious buildings and choir practices,^[59] and hospitals and other healthcare settings.^[60] A superspreading event in a Skagit County, Washington, choral practice resulted in 32 to 52 of the 61 attendees infected.^[61][5]

An existing model of airborne transmission (the Wells-Riley model) was adapted to help understand why crowded and poorly ventilated spaces promote transmission,^[5] with findings supported by aerodynamic analysis of droplet transfer in air-conditioned hospital rooms.^[46] Airborne transmission also occurs in healthcare settings; long-distance dispersal of virus particles has been detected in ventilation systems of a hospital.^[60]

Some scientists criticized public health authorities, including the WHO, in 2020 for being too slow to recognize airborne (aerosol) transmission of COVID-19 and to update their public health guidance accordingly.^{[62][63][64][65]} By mid-2020, some public health authorities had updated their guidance to reflect the importance of airborne transmission.^[10][66] The WHO updated it only by 23 December 2021.^[65][11]

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

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.^{[36][11][16][19][24][34]} 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."^[19]

Each contact with a surface contaminated with SARS-CoV-2 has less than a 1 in 10,000 chance of causing an infection.

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.[77]

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.^[19] Studies have not found any viable virus in breast milk.^[19] Breast milk is unlikely to spread the COVID-19 virus to babies.^[78][79] 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.^[79][19]

Food and water

No evidence suggests that handling food or consuming food is associated with transmission of COVID-19.

The COVID-19 virus has not been detected in drinking water.^[82] Conventional water treatment (filtration and disinfection) inactivates or removes the virus.^[82] COVID-19 virus RNA is found in untreated wastewater,^{[82][22][83][a]} but there is no evidence of COVID-19 transmission through exposure to untreated wastewater or sewerage systems.^[82] There is also no evidence that COVID-19 transmission to humans occurs through water in swimming pools, hot tubs, or spas.^[82]

Other

While SARS-CoV-2 RNA has been detected in the urine and feces of some persons infected with COVID-19,

COVID-19 is more infectious than influenza, but less so than measles.^[34] Estimates of the number of people infected by one person with COVID-19—the basic reproduction number (R₀)—have varied. In November 2020, a systematic review estimated R₀ of the original Wuhan strain to be approximately 2.87 (95% CI, 2.39–3.44).^[90] 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.^[91]

Temperature is also a factor that affects the transmissibility of the virus. At elevated temperatures and low virus concentration rates the virus is in its weak state^[92] and the spreading of it is strenuously. At low temperatures and excessive virus concentration rates the virus is in its robust state^[92] and the spreading of it is unchallenged.

Effect of face masks and face shields

None of the protective face masks and face shields completely block the escape of particles projected by a