To develop effective public health and infection prevention measures to break chains of transmission it is critical to understanding how, when, and in what types of settings SARS-CoV-2 spreads between people.
The transmission of COVID-19 is mainly occurring from symptomatic people and can occur before they develop symptoms when they are near others for an extended period. Also, someone who is asymptomatic can pass the virus to others. SARS-CoV-2 the virus that causes COVID-19 is mainly spread from person-to-person. The possible modes of transmission for SARS-CoV-2, including contact, droplet, airborne, and fomite transmission. Infection with SARS-CoV-2 primarily causes respiratory illness ranging from mild disease to severe disease and death, and some people infected with the virus never develop symptoms.
The transmission of SARS-CoV-2 can occur through direct, indirect, or close contact/ This occurs through the infected peoples’ secretions such as saliva and respiratory secretions or their respiratory droplets, which are expelled through coughing, sneezing, talking or singing. Droplets <5μm in diameter are called droplet nuclei or aerosols. Respiratory droplets are >5-10 μm in diameter. Respiratory droplet transmission happens when a person is within 1 meter of an infected person who has respiratory symptoms or who is talking or singing. If the infected respiratory droplets that include the virus reach their mouth, nose, or eyes it can lead to infection. Indirect contact transmission involves contact of a susceptible host with a contaminated object or surface (fomite transmission).
Airborne transmission is “the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time.” Airborne transmission can occur when medical procedures, called aerosol-generating procedures, generate very small droplets called aerosols. It is believed that SARS-CoV-2 can spread through aerosols without aerosol-generating procedures, such as indoor settings with poor ventilation. Outbreaks in indoor crowded spaces have suggested the possibility of aerosol transmission, combined with droplet transmission.
The physics of exhaled air and flow physics have caused hypotheses about possible mechanisms of SARS-CoV-2 transmission through aerosols. These theories suggest that “1) a number of respiratory droplets generate microscopic aerosols (<5 µm) by evaporating, and 2) normal breathing and talking results in exhaled aerosols.” Therefore, a susceptible person could inhale aerosols that contain the virus and become infected.
Respiratory secretions or droplets from infected individuals can land on objects and surfaces, creating fomites (contaminated surfaces). Environmental contamination suggests people can be infected by touching these surfaces and touching their eyes, nose, or mouth before cleaning their hands. Viable SARS-CoV-2 virus and/or RNA detected by RT-PCR can be found on those surfaces for hours to days, depending on the ambient environment and the type of surface, at a high concentration where COVID-19 patients were being treated. Therefore, transmission can occur indirectly through touching surfaces in the immediate environment or objects contaminated with virus from an infected person, followed by touching the mouth, nose, or eyes. Fomite transmission is a likely mode of transmission for SARS-CoV-2, due to the findings of environmental contamination.
Knowing when an infected person can spread SARS-CoV-2 is just as important as how the virus spreads. WHO has recently published a scientific brief outlining evidence that suggests SARS-CoV-2 RNA can be identified in people 1-3 days before their symptoms begin, with the highest viral loads, as measured by RT-PCR, observed during the onset of symptoms, followed by a decline over time. The duration of RT-PCR positivity is 1-2 weeks for asymptomatic people and up to 3 weeks or more for people with mild to moderate symptoms. It can be much longer for patients with severe symptoms of COVID-19.
SARS-CoV-2 is primarily transmitted between people through respiratory droplets and contact routes, and that transmission of COVID-19 is occurring from people who are pre-symptomatic or symptomatic to others in direct contact for a continuous period. SARS-CoV-2 is also transmitted through aerosolization in healthcare facilities where aerosol-generating procedures are used. People who are infected and are asymptomatic can also transmit COVID-19.
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