What is viral shedding

Say, you come in contact with a virus—something that could cause an ordinary illness like the common cold. Now whether or not you will get this viral infection will come down to a few things:

How widely the virus spreads in your body
How potent it is
And your body's immune response to it

The process by which a virus spreads inside the human body (viral pathogenesis) is an elaborate one:

First, the virus needs to enter the body through specific points (for example, the mucosal linings of the nose, throat or eyes)
Second, it needs to attach itself to specific cellular receptors of the human body
Third, it takes over the resources of the host cell to replicate or multiply into the system to cause disease.

When the virus moves from one infected cell to another, from one infected body tissue to another and eventually from an infected person into the surroundings where it may infect another a healthy person, this is known as viral shedding.

There are several ways in which viral shedding can occur. For example, through sneeze or cough droplets (droplet transmission) or the faeces of an infected person viral shedding.

(It's important to remember that viral shedding by itself does not always lead to new infections—there has to be enough virus present (viral load) in the given sample and the virus will need a viable host nearby. For example, we know that people with polio continue to shed the poliovirus in their poo throughout their lives. But this usually doesn't cause fresh infections, because India has eradicated this disease.)

Viral shedding in the context of COVID-19

COVID-19 is a respiratory infection that has spread throughout the world. In just over six months, it has affected more than 13.5 million individuals and killed over 584,000 of them globally. Naturally, this has put a spotlight on the infectious nature of the SARS-CoV-2 virus, the pathogen responsible for this new coronavirus infection.

The rate at which COVID-19 has spread to all parts of the world is an example of how effectively the SARS-CoV-2 virus has jumped from one host body to another—it is also proof that different strategies to limit the spread of the disease either came too late or weren't effective enough to be successful yet.

Researchers studying this phenomenon to understand the infectious nature of COVID-19 believe that it is during viral shedding that the symptoms begin to worsen, and the disease is in its most contagious form. However, research also indicates that people can be infectious even when symptoms haven’t developed in the early stages of contracting the disease.

How does viral shedding occur?
Effect of viral shedding on disease transmission
Viral shedding and containment strategies for COVID-19

Doctors for What is viral shedding

How does viral shedding occur?

According to the description in the book Medical Microbiology, viral pathogenesis is the process through which an infection metamorphoses into a disease in an individual. As explained above, the process is elaborate—it takes place through the following steps:

The virus attaches itself to its specific entry point in the human body (ACE2 receptors in the case of COVID-19). It enters the healthy cells of the body through these receptors. This is important because this step allows the virus to get inside the cell undetected by the body's immune system.
The virus begins to make copies of itself (replicate) using the healthy cell's resources and materials
The virus spreads to different target locations or organs where disease manifests
The virus moves to different parts of the body from where it begins to shed and move into the outside environment

The route which SARS-CoV-2 virus prefers to take to enter the human body is through the respiratory system, via droplet transmission, although different kinds of viruses may have different points of access in the body. Viral shedding can occur at different sites in the body, for example, through bodily fluids like saliva, blood and semen.

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Effect of viral shedding on disease transmission

Viral shedding is what makes an infected individual contagious—they have to shed the virus to enable human to human transmission. In the case of COVID-19, the duration of viral shedding has been debated in different studies, with an early study in Germany indicating those who had contracted the infection shed the virus heavily in the first few days of getting infected.

Read more: How does COVID-19 spread

This proved to be critical in understanding why the disease spread so quickly from Wuhan, where it is believed to have originated, to different parts of the world undetected.

Later studies produced evidence of viral shedding for a much more prolonged duration. A study published in the International Journal of Infectious Diseases by Chinese researchers suggested that the median duration of viral shedding was 17 days, which is considerably high.

In the 147 COVID-19 patients included in the study, factors such as the highest body temperature, time from the onset of symptoms to admission and the duration of stay in the hospital played a part in prolonging the duration of viral shedding.

Another study published earlier in the scientific journal Nature showed that those infected with COVID-19 showed the highest levels of viral shedding just before developing symptoms, based on an assumed incubation period of 5.2 days. The Chinese study looked at 94 patients admitted with COVID-19 and the highest rate of infectiousness was 0-2 days before the onset of symptoms, and contagiousness appeared to decline in seven days.

The study also concluded that the infectiousness of the SARS-CoV-2 virus appeared to be similar to the influenza virus rather than the coronavirus that causes severe acute respiratory syndrome or SARS (the last recorded case of SARS was in 2004).

Viral shedding and containment strategies for COVID-19

The above-mentioned Chinese study on the transmissibility of COVID-19 explained that SARS was much easier to detect, so its rate of transmission could be reduced by effective containment strategies like isolating the patients and quarantining their contacts. It helped that the infectiousness of the disease was at its highest 7-10 days after the onset of symptoms.

In the case of COVID-19, such containment strategies appear to have fallen short also because the process of viral shedding or the infectiousness of the disease is the highest shortly before the onset of symptoms.

So even with a low reproductive number (R0 or R-naught), strategies to contain the SARS-CoV-2 infection are less likely to be successful if even a third of the transmissions happen before the onset of symptoms. Contact tracing could have been one of the most effective strategies to control the spread of the disease, but primary cases needed to be identified quickly.

(R0 shows how many people an average infected person gives the virus to. This varies from virus to virus. For example, in the case of measles, it's 12-18 and in the case of the common cold, R0 is two to three.)

This is why—the study suggests—Hong Kong, China and South Korea were able to limit cases initially through effective contact tracing strategies, to identify index cases before the onset of symptoms. The scientists go on to explain that contact tracing could help during superspreader events as well.

Remember, viral shedding is at its highest before and during the onset of COVID-19 symptoms, so an infected person may transmit COVID-19 even before they realise they have the disease.