The Viral Load: The X-Factor predicting disease severity in COVID-19


Anyone can suffer from vastly different severities of disease, depending on the viral load they are exposed to.

Coronavirus Particles. The amount of these makes an impact when it comes to severity of disease. Credit: Maurizio de Angelis

My younger brother and I have had two widely different experiences of COVID-19 illness. We both self-isolated at the same time, as we are sharing a flat. He had no symptoms during this entire period, apart from a very mild cough for half a day.

Conversely, I am now four weeks into my COVID-19 illness. The course of my illness was complicated by pneumonia, for which I have taken two antibiotics. We are both NHS doctors in the United Kingdom, and I run and cycle regularly whereas he doesn’t. Neither of us have any background medical problems.

Why did we have such different experiences of COVID-19?

Patients with two different viral loads, causing different courses of illness. Credit: Tgraph

There are other factors at play here, regardless of age, immune status or pre-existing health conditions:

  • Viral load: The number of viral particles being carried by an infected individual and shed into their environment and Infectious Dose: The amount of viral particles needed to cause an infection
  • Host genome: Each person has a genetic profile which might make them more or less susceptible to a particular disease. Unfortunately, this can’t be changed.
  • Gut microbiome: The vast ecosystem of organisms that live in our digestive system and contribute to how we respond to diseases.
  • Host immune system: Relating to the genome, we each have an immune repertoire which may make us more or less likely to react to coronavirus. This impacts things like the cytokine storm, which is an overreaction of the immune system

The viral load is a key factor when it comes to disease severity:

“The viral load is a measure of how bright the fire is burning in an individual, whereas the infectious dose is the spark that gets that fire going” Edward Pinker, London School of Hygiene and Tropical Medicine

People who have a higher viral load will be shedding more particles of COVID-19, and are therefore more likely to infect other people. The exposure is also affected by the point in the infected individuals’ illness when you come in contact with them. Peak shedding confers the highest risk with reports that people are shedding 1,000 times more virus during this time.

COVID-19 viral shedding timeline. Credit: Tgraph

I may have caught the virus whilst doing a laser retina procedure on a patient (neither of us were wearing face masks or PPE) a week before I started getting symptoms. This was a long procedure, and we do know that Ophthalmologists and ENT Doctors are at considerably higher risk of getting the disease, due to the time and proximity between the patient and the doctor. Dr Li Wenliang the whistleblower for the pandemic, was an Ophthalmologist.

My younger brother would have likely caught the virus from me. As soon as I developed symptoms, I tried to stay as distant from him as possible within our self-isolation period, to ensure as low a dose as possible was given to him.

Two very different courses from the same virus. The question is — does a higher viral load mean worse disease?

“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” Willem van Schaik, professor in Microbiology and Infection at the University of Birmingham

This would certainly fit with what we know about other viruses. For example, a higher dose of Influenza-A has been associated with worse symptoms in previous studies. Analysis of patients in Hong Kong during the previous SARS pandemic also found worse symptoms associated with higher viral loads. This appears to be the case with COVID-19 as well. In hospitalised patients in Nanchang, there was a strong link between the amount of virus found in the nose and the severity of disease.

The relationship between number of particles and the outcome. Credit: Tgraph

The disproportionate number of healthcare workers becoming sick from COVID-19 seems to be clear, and a cause for concern. Health workers are more likely to be exposed to higher viral loads, and this can even have a cumulative effect at first exposure to the virus.

One thing is for certain, COVID-19 is a wily beast who does not play by the rules, and the evidence isn’t as concrete as you would hope. In Lombardy, tracing contacts of infected people found that there was no difference in viral loads between those with symptoms and those without. In Guangzhou, there was little difference in viral load between milder and more severe cases. We still have a lot of learning to do about how this disease manifests, and what other factors might be at play.

Rendering of a single Coronavirus. Credit: Getty

Regardless of the degree that viral load impacts severity of illness, we need to take every step to prevent ourselves getting infected, and therefore pass the virus onto others. This is where lockdown measures, and personal protective equipment (PPE) are crucial to stop the spread of this uniquely nasty virus.

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