It is believed that the novel coronavirus made its way to humans from bats with the pangolin as the intermediary host. It originated in Wuhan, the capital city of Hubei province in the People’s Republic of China.
During its transmission, it underwent a natural process of evolution and we are left dealing with a new form of the coronavirus about which we have no previous data.
Within less than 100 days of its discovery, this virus attained pandemic proportions, with most countries in the world enforcing a lockdown without much success.
It does so by two modes of transmission:
- Through air
- Through commonly touched surfaces
Coronavirus transmission in humans through the air either as aerosols or through droplets
The infected person could throw the coronavirus into the air through two forms: as aerosols or as droplets.
Aerosols are minute particles suspended in the atmosphere, which contain the coronavirus. However, the size of an aerosol particle is less than 5μ and it is emitted when the person talks or coughs or sneezes.
Droplets are bigger in size more than 5µ are thrown into the air when the person coughs or sneezes.
Transmission through aerosols
According to a study conducted by National Institute of Allergy and Infectious Diseases’ Laboratory of Virology in the Division of Intramural Research in Hamilton, Montana and published in the New England Journal of Medicine on March 17, 2020, the infectious virus could remain airborne for up to three hours in fine particles known as aerosols.
The coronavirus can be present in the air as aerosol or in droplets thrown into the air by an infected person by coughing and sneezing.
Airborne transmission (aerosol) is different from droplet transmission
As mentioned above, the virus in the aerosol can stay in the air for up to three hours. Whether the virus can infect another person during this time and in this form is a matter of debate.
Till today, we have scientists on both sides claiming opposite views and the matter stands unresolved until some concrete evidence emerges.
However, as of now, airborne transmission is ruled out except in certain conditions where the hospital procedures or support treatments that generate aerosols are performed. Such procedures include endotracheal intubation, bronchoscopy, administration of nebulized treatment.
According to the World Health Organization (WHO), “In an analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported”.
Transmission through droplets
However, transmission through droplets of saliva released through coughing or discharge from the nose when the infected person sneezes is established.
These droplets can infect another healthy person if present within one meter of the infected person. These droplets are typically 5-10 microns in size.
From an infected person, there can be up to 40,000 droplets of saliva in one sneeze of average force, which travels at the speed of over 320 kmph.
These droplets are laden with viruses and bacteria with each droplet containing up to 2 million viruses.
It is easy to see how a healthy person in close vicinity can get infected with these droplets landing on his nose, mouth, and eyes. The person in the close vicinity can even inhale these droplets suspended in the air.
When a few of these infected droplets reach the respiratory tract of a healthy person, the coronavirus invades the cells of the healthy person who now becomes the host.
This virus has a tremendous capacity to multiply and colonize inside the host cell. Within a week, the virus multiplies to billions and the person would start showing the symptoms of COVID-19.
According to research by scientists at the Massachusetts Institute of Technology, coughing spreads the droplets as far as six meters, and sneezing can send the droplets as far as eight meters. These droplets can stay floating in the air for up to 10 minutes.
The 6-foot (2 meters) guideline is more of a rough estimate than a hard and fast established fact.
The infected person is coughing and spreading the coronavirus within the first 4 to 5 days after he has been infected. He is least suspected of being infected at that time because he is not showing any symptoms and even if randomly checked, his blood test comes out negative.
How are health workers at risk in spite of PPE?
In spite of personal protective equipment (PPE), health workers are at risk of catching the virus from infected patients.
Health care workers might also collect tiny infected droplets coughed and sneezed out on their protective gear when looking after infected patients.
They might resuspend these big and small droplets into the air when they take off their protective gear after duty.
They then become exposed to the virus by breathing in the suspended infected droplets.
Transmission through surfaces
The viral particles sneezed or coughed out from the respiratory tract of an infected individual land on a surface. It could be anything; tabletop, telephone instrument, door handle.
Then, another person touches that object, and then with the same hand touches his nose, mouth or eyes. The virus is then able to gain entry into the body of that person via the mucous membranes, infecting that person.
According to a study, people touched their faces an average of about 23 times an hour. Forty-four percent of these touches were to the mouth, nose, and eyes. Being moist, these are the known entry points of our body for infection.
There is enough evidence that contact with contaminated surfaces can transmit the virus.
However, the virus has a different lifespan on different surfaces such as cardboard, plastic, metal, fabric, and copper. It tends to live longer on some surfaces such as plastic and metal for as much as 72 hours, while it lasts for only 10 minutes on your hands.
However, people are much more likely to be infected by close contact with an infected person than by touching a contaminated surface.
There’s a certain amount of viral concentration that is required to infect a person. It depends on the potency of the virus. You will not be infected if you had just one viral particle on your finger.
Some viruses are very potent and they can infect you with just 10 particles on your finger, while with others you may need millions of these virus particles.
A person who is at high risk catches the infection more easily than a young healthy person with no comorbidities.
Less concentration of viral particles makes it less likely that you will be infected. That is why the amount of concentration of the virus on a surface is important.
The viral load of coronaviruses on inanimate surfaces to be a source of infection is still not known.
To sum up corona transmission
The SARS-CoV-2 virus that causes the COVID-19 disease is mainly transmitted via the droplets that are emitted when the infected person coughs, sneezes and talks.
These droplets, being heavy travel for a distance of one meter only and then fall on the floor or surfaces.
You can get infected by inhaling the infected droplets from the air if you are within that one-meter distance when the droplet is suspended in the air.
You can also be infected by touching a contaminated surface and then touching your eyes, nose or mouth by which the virus can gain entry through the mucosal lining of these body parts into your respiratory system.