Transmission of COVID-19

The transmission of the virus is carried out 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.^[6] If the droplets are above a certain critical size, they settle faster than they evaporate, and therefore they contaminate surfaces surrounding them.^[6] Droplets that are below a certain critical size, evaporate faster than they settle; due to that fact, they form nuclei that remain airborne for a long period of time over extensive distances.^[6]


Infectivity can begin four to five days before the onset of symptoms.^[7] Infected people can spread the disease even if they are pre-symptomatic or asymptomatic.^[8] 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.^[8] 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.^[9][8]


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 indirectly by touching a contaminated surface or object before touching their own mouth, nose, or eyes,^[8][19] though strong evidence suggests this does not contribute substantially to new infections.^[12] 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.^[20] Although it is considered possible, there is no direct evidence of the virus being transmitted by skin to skin contact.^[16] Transmission through feces and wastewater have also been identified as possible.^[21] The virus is not known to spread through urine, breast milk, food, or drinking water.^[19][22] It very rarely transmits from mother to baby during pregnancy.^[16]