How is COVID-19 transmitted?
Two factors facilitated the initial rapid spread of COVID-19 in Wuhan: i) a population of 11 million inhabitants that increased the chance of person-to-person contact, and ii) the city’s busy transportation hub, which increased the likelihood of exporting cases to other locations. Despite Chinese containment measures, COVID-19 has grown into a full-blown pandemic.
The R factor, a virus’ basic reproductive number, is referred to as R0 – the average number of people someone carrying the virus will infect. The higher the R0, the faster an epidemic can spread. At the start of the pandemic, R0 for SARS-CoV-2 was estimated at 2.0 to 2.5, indicating that one patient could transmit the virus to two (or slightly more) other people. The doubling time for COVID-19 cases is estimated at three to six days.
A transmission electron microscopic image of an isolate from the first US case of COVID-19. Credit: CDC.
The virus is transmitted primarily through droplets 5–10 μm in diameter, released when an infected person coughs, sneezes, talks, or even exhales. These airborne droplets can attach to the respiratory tract mucosa or conjunctiva of another person. They can also settle on surfaces or fomites and be transferred to another person upon contact. SARS-CoV-2 is more stable on plastic and steel (up to three days) than on cardboard (up to one day) or copper. Viral transmission is possible if someone touches their face, eyes, nose, or mouth following contact with contaminated surfaces or fomites.
Transmission may also occur through aerosols, which are particles smaller than 5 μm. SARS-CoV-2 remains viable in these particles for up to three hours. Aerosol transmission is a serious risk to health care workers during procedures such as intubation, bronchoscopy, suctioning, turning a patient to the prone position, or disconnecting a patient from the ventilator.
A single cough or a sneeze emitted by an infected person may spread as many as 200 million virus particles.
Some experts estimate that exposure to as few as 1,000 SARS-CoV-2 particles can cause infection. One releases about 3,000 respiratory droplets that travel at 50 miles per hour; most are large and quickly fall to the ground, but many remain airborne and can travel across a room in a few seconds. A sneeze releases about 30,000 droplets traveling up to 200 miles per hour, most of which are small and travel great distances. A single cough or a sneeze emitted by an infected person may spread as many as 200 million virus particles.
In contrast, a single breath releases only 50 to 5,000 droplets, most of which travel at low velocity and drop quickly. Because breath is expelled at low force, viral particles residing in the lower respiratory areas are not expelled – meaning that breathing may release as few as 20 to 30 viral particles per minute. Speaking increases the release about tenfold (200 virus particles per minute), so five or more minutes of face-to-face conversation could lead to infection.
But infection with SARS-CoV-2 depends not only on dose, but also exposure time. If an infected person coughs or sneezes directly toward someone, they can inhale 1,000 viral particles in a few minutes. If someone enters a room shortly after an infected person coughs or sneezes, it may take only a few breaths – whereas if they simply occupied a room where an infected person was breathing, it might take 50 minutes or longer to inhale an infectious dose.