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Explainer: What is a viral load and does it impact on the severity of Covid-19 symptoms?

Concerns were raised on social media in relation to the viral load that would be contracted by healthcare workers.

A mural of the virus has appeared in Rathmines in Dublin.
A mural of the virus has appeared in Rathmines in Dublin.
Image: Leah Farrell

PEOPLE ACROSS THE country have been severely restricting their movements for over two weeks now following Government measures aimed at slowing the spread of the Covid-19 coronavirus. 

The goal is to reduce contact and transmission of the virus but those working on the frontline of healthcare such as doctors and nurses, and those drafted in from other sectors such as defence and education to support healthcare staff, do not have the luxury. 

They are working in hospital and healthcare settings where the virus is present and so to reduce the risk of that cohort of the population contracting the virus, significant resources have been put behind securing and rolling out personal protective equipment, or PPE. 

On Sunday the first delivery of PPE from China – one of ten flights which were scheduled to land over the past four days – arrived on board an Aer Lingus flight and were due to be dispersed to over 2,000 locations around the country that same day. 

In the meantime, comments across social media and from healthcare workers themselves, have pointed to concerns that those working on the frontline who contract the virus will experience more severe symptoms than the general population due to the “viral load” being greater. 

But what exactly is the viral load, which is different to the initial infection source known as the infectious dose, and is there any truth behind social media claims that healthcare workers will experience worse symptoms than the general population? 

There is no simple yes or no answer to this question as Kim Roberts, a virologist at Trinity College Dublin, explained. 

“If you’re exposed to a large amount of a virus it can lead to a more severe infection but the severe infection is multifactorial and multiple things play a role.

“Your own health and immune system come into play and there is a host of other things. That’s where PPE comes in and that PPE is all about reducing the chance that the virus they are in contact with will actually infect them.”

Let’s start from the beginning – what is a viral load?

A viral load is the total amount of virus that a person has inside them. An easy way to think about a viral load is through the treatment of another virus – the Human Immune Virus (HIV). 

In recent years, new retroviral drugs have been developed which have been able to almost completely eliminate the presence – or viral load – of HIV in the bloodstream of a person who has contracted it.

If a person was to stop taking those drugs, the human immune virus would begin to replicate at a rapid pace and the presence of it in a person would multiply i.e the viral load would increase, putting pressure on the human immune system.  

Similarly, when someone contracts the Covid-19 virus, that virus begins to replicate and strengthen its hold in a person’s body. That’s when the immune system kicks in and begins its efforts to fight it off.

A person’s ability to fight the replication of the virus then depends on how healthy they are and how strong their immune system is.

The weaker the immune system, the greater the chance it has to replicate and so the greater the chance of the viral load increasing, and symptoms associated with the virus becoming more severe.

But this is different from the initial ‘infectious dose’. 

The infectious dose refers to the number of virus particles exposed during transmission and the amount of those which is needed for the virus to infect a person. But this has incorrectly been conflated with the meaning of the ‘viral load’ recently. 

When a virus is transmitted from one person to another, it means hundreds, or thousands of particles in the case of some viruses, are transmitted from one person to another – usually through a cough or a sneeze. 

Many of those particles will be suppressed by a person’s immune system once they reach the body – but some will make it through and subsequently infect a cell, where it can begin to replicate. 

As healthcare workers are working in closer proximity to people with the virus than those in the general population, they are exposed to these particles more often – and in greater amounts – so the likelihood of infection increases. 

“The minimum infectious dose is how many virus particles you can be exposed to in order to trigger a viral infection, and we don’t know what it is in the case of Covid-19 yet. It varies from one virus to another,” Roberts explained. 

And when a person, hospitalised due to the virus, coughs or sneezes, they release much greater numbers of virus particles than those in the general population who might be experiencing milder symptoms. 

“It’s all about probability and it’s the probability that you are exposed to enough of the virus so that one particle can fight through those respiratory and other barriers, other natural barriers that we have, so it can get into a cell,” Roberts said. 

“All it takes is one virus particle to get to the cell and it will replicate but your body still has a system to suppress the virus.

It’s down to the immune system of that person because they might have had a small infectious dose but their immune system might not have had the resources to stop it.

And as Dr Michael Skinner of Imperial College London points out here, “It is unlikely that higher doses that would be acquired by being exposed to multiple infected sources would make much difference to the course of the disease or the outcome.”

What does this mean for healthcare staff?

It means the severity of the symptoms experienced by anyone who contracts the virus, including healthcare workers, is down to the individual immune response, and other factors such as weakened immune systems, underlying conditions etc. 

The likelihood of contracting the virus in the first place, however, is heightened, but employing PPE, repeated hand-washing, and other protocols can significantly reduce that from happening. 

It is also worth noting that while one in four cases – 841 of the 3,849 confirmed cases – of the virus in Ireland have been identified in healthcare workers, much of those were cited as occurring due to travel, with fewer than a quarter of those workers contracting the virus in a healthcare setting. 

The HSE and Department of Health have issued guidelines for frontline staff around using personal protective equipment, and this is the easiest way to reduce the likelihood of healthcare workers contracting the virus. 

“Good infection prevention and control (IPC) practice, supported by appropriate use of PPE is important to minimise the risk of exposure to occupational infections for healthcare workers (HCWs),” a spokesperson said. 

There is also a range of videos here demonstrating how PPE should be used.  

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