COVID-19 is mainly transmitted from person to person through inhaling air contaminated by droplets/aerosols and small airborne particles containing the virus. Infected people exhale those particles as they breathe, talk, cough, sneeze, or sing. Transmission is most likely at closer range but can also occur over longer distances, particularly indoors.
The virus spreads through virus-laden fluid particles, or droplets, which are created in the respiratory tract, and they are expelled by the mouth and the nose. There are three types of transmission: "droplet" and "contact", which are associated with large droplets, and "airborne", which is associated with small droplets. If the droplets are above a certain critical size, they settle faster than they evaporate, and therefore they contaminate surfaces surrounding them. Droplets that are below a certain critical size, generally thought to be <100 micrometres (μm) diameter, evaporate faster than they settle; due to that fact, they form respiratory aerosol particles that remain airborne for a long period of time over extensive distances.
Infectivity can begin four to five days before the onset of symptoms. Infected people can spread the disease even if they are pre-symptomatic or asymptomatic. 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. Current evidence suggests a duration of viral shedding and the period of infectiousness of up to ten days following symptom onset for people with mild to moderate COVID-19, and up to 20 days for persons with severe COVID-19, including immunocompromised people.
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. Additionally, COVID-19 research has redefined the traditional understanding of how respiratory viruses are transmitted. 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. Aerosols are highest in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close, but airborne transmission can occur at longer distances, mainly in locations that are poorly ventilated; in those conditions small particles can remain suspended in the air for minutes to hours.
The number of people generally infected by one infected person varies, but it is estimated that the R0 ("R nought" or "R zero") number is around 2.5. The disease often spreads in clusters, where infections can be traced back to an index case or geographical location. Often in these instances, superspreading events occur, where many people are infected by one person.
A person can get COVID-19 indirectly by touching a contaminated surface or object before touching their own mouth, nose, or eyes, though strong evidence suggests this does not contribute substantially to new infections. Transmission from human to animal is possible, as in the first case, but the probability of a human contracting the disease from an animal is considered very low. Although it is considered possible, there is no direct evidence of the virus being transmitted by skin to skin contact. Transmission through feces and wastewater have also been identified as possible. The virus is not known to spread through urine, breast milk, food, or drinking water. It very rarely transmits from mother to baby during pregnancy.
Infectious period
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. 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. Initial reports suggested that this early transmission was restricted to the two-to-three day time window, but an author correction later acknowledged that transmission could begin four to five days before symptom onset.
People are most infectious shortly before and after their symptoms begin—even if mild or non-specific—as the viral load peaks at this time.
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 few transmission events are observed after five days. 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.
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.
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.
Asymptomatic transmission
People who are asymptomatic do not show symptoms but still are able to transmit the virus. At least a third of the people who are infected with the virus do not develop noticeable symptoms at any point in time.Multiple sources:
Asymptomatic carriers tend not to get tested.
Persons with asymptomatic COVID-19 infection can have the same viral load as symptomatic and presymptomatic cases, and are able to transmit the virus. However, the infectious period of asymptomatic cases has been observed to be shorter with faster viral clearance.
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). Spread occurs when the particles are emitted from the mouth or nose of an infected person when they breathe, cough, sneeze, talk, or sing. Human breath forms a roughly cone-shaped plume of air; in an infected person, the breath carries out the virus-containing droplets. 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. But breath contains many droplets that smaller than 100 μm in size, and these can stay suspended in the air for at least minutes and move across a room. There is evidence that infectious SARS-CoV-2 survives in aerosols for a few hours. 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. 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.
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. 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, 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.
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. The jet's reach is nearly 22 ft in 18.5 seconds and 25 ft in 22 seconds. The shape of the expelled particles is conical, with a spreading angle of 23 degrees. The trajectory of the turbulent jet is inclined due to the inclination angle of the nose. Smaller droplets travel a considerable distance as freely suspended tracers and may still get reflected and follow the turbulent cloud. 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.
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.
The risk of transmission from all size droplets and aerosols is lower in indoor spaces with good ventilation. The risk of outdoor transmission is low.
Transmission events occur in workplaces, schools, conferences, sporting venues, dormitories, prisons, shopping facilities, and ships, as well as restaurants, passenger vehicles, religious buildings and choir practices, and hospitals and other healthcare settings. A superspreading event in a Skagit County, Washington, choral practice resulted in 32 to 52 of the 61 attendees infected.
An existing model of airborne transmission (the Wells-Riley model) was adapted to help understand why crowded and poorly ventilated spaces promote transmission, with findings supported by aerodynamic analysis of droplet transfer in air-conditioned hospital rooms. Airborne transmission also occurs in healthcare settings; long-distance dispersal of virus particles has been detected in ventilation systems of a hospital.
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. By mid-2020, some public health authorities had updated their guidance to reflect the importance of airborne transmission. The WHO updated it only by 23 December 2021.
Medical procedures designated as aerosol-generating procedures