Transmission of COVID-19

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. 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

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