What is viral load and does it affect Covid-19?

Increasingly hearing about ‘viral load’ on the news and not sure what it means for you? We share what the experts are saying and what it means for living well during the coronavirus outbreak.

What is viral load?

Viral load is a medical term that has long been used to describe how much virus is detected in a sample of blood or plasma taken from an infected patient. Put simply, our viral load is how much of a specific virus we have in our body.

Along with terms such as ‘social distancing’ and ‘self-isolation‘, it’s part of the new vocabulary that we’re all learning since the outbreak of coronavirus. Prior to this outbreak, it was most commonly used to measure the effectiveness of HIV medication. A HIV patient who responds well to their treatment, for instance, will see a dramatic reduction in their viral load, meaning they cannot pass the disease on to others.

Why does viral load matter?

To understand the importance of viral load, we need to understand how our bodies fight off a virus once infected.

“An infection can start with just a small number of articles (the ‘dose’),” says Dr Michael Skinner, Reader in Virology at Imperial College London.

“When that dose reaches our respiratory tract, one or two cells will be infected and will be re-programmed to produce many new viruses within 12 to 24 hours. The new viruses will infect many more nearby cells and the whole process goes round again, and again, and again.”

This is when our immune system kicks in with what is known as our innate immune response. This immune response is not the acquired immune response that we’re generally more familiar with (i.e., the release of disease-specific antibodies). Rather, this first response is more general and is designed to slow the spread of the virus while our body is working on these essential antibodies, that will stop and finally clear the infection. The innate immune response involves flooding the body with small proteins (known as interferon and cytokines), which have the side effect of producing many of the well known viral symptoms such as fever, headaches and muscle pain.

“With Covid-19, these two arms of the immune system (innate and acquired) obviously work well for 80% of the population who recover from a more or less mild influenza-like illness,” Dr Skinner explains.

If our initial ‘dose’ or viral load is too great, however, the immune system can struggle to slow the spread for long enough.

“Under such circumstances, the virus receives a massive jump start, leading to a massive immune response,” says Dr. Skinner.

It’s the mass inflammation caused by this huge immune response (known to medics as a ‘cytokine storm’) that can require many to need intensive care treatment and can greatly increase the risk of death.

Does viral load affect Covid-19 symptoms?

Though it will take some time for Covid-19 research to catch up with demand, work on two other strains of coronavirus suggests that viral load may play a key role.

“On the basis of previous work on SARS and MERS coronaviruses, we know that exposure to higher doses are associated with a worse outcome and this may be likely in the case of Covid-19 as well,” says Willem van Schaik, Professor of Microbiology and Infection at the University of Birmingham.

This hypothesis also appears to be confirmed in the early reports from China which suggest there is a higher viral load in patients with a more severe disease.

Can our viral load accumulate if exposed to more than one infected individuals?

Some researchers believe that if we’re surrounded by many infected individuals, there is a higher number of viral particles in the air and we may end up with an accumulated higher viral load at the point of infection.

“This means that health care workers that care for Covid-19 patients are at a particularly high risk as they are more likely to be exposed to a higher number of viral particles, particularly when there is a lack of personal protective equipment (PPE) as is reported in some UK hospitals,” Professor Van Schaik explains.

Evidence also shows, however, that once infected it’s unlikely that you can increase your viral load by getting infected from other people.

“There is no evidence for any suggestion that if everyone in a family is already sick they can reinfect each other with more and more of the virus, says Professor Wendy Barclay, Head of Department of Infectious Disease at Imperial College London

“In fact, for other viruses once you are infected it’s quite hard to get infected with the same virus on top.”

How can we limit our viral load?

It’s worth recognising that, for most, exposure to a high dose of coronavirus is unlikely.

“Situations we should be concerned about are potential high dose exposure of clinical staff conduction procedures on patients who are not known to be infected,” says Professor Van Shaik.

He hopes that in these cases testing patients will become procedure (even if they display no symptoms), and that some non-urgent procedures and operations will be postponed to protect staff.

In the case of necessary procedures (such as in A&E and in maternity wards) PPE equipment is absolutely necessary to protect our healthcare workers.

Those caring for loved ones at home are also at risk. This is why the government recommends sleeping in different rooms and using different bathrooms if possible.

The rest of us can reduce our risk by social distancing as per the government guidelines. Analysis published in The Lancet has shown that Covid-19 patients are most infectious during the early stages of the illness when many show little or no symptoms. This is why it’s so important to maintain our distance from those outside our household even if they report feeling well.

Finally, when battling a respiratory virus Professor Barclay explains “it’s all about the size of the armies on each side of the battle.”

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