Explainer: what is viral load?

The COVID-19 pandemic has introduced us to a raft of new terminology in the past two years, with important implications for the safety of our population.

Now that the majority of Australians are fully vaccinated, it’s important to understand the concept of viral load.

What is viral load?

This term refers to the amount of viral particles inside the body of someone who has contracted the SARS-CoV-2 virus. When a person is infected, the virus hijacks the cells’ mechanisms in order to replicate itself – and infect more cells. The more this happens, the greater the viral load.

Does viral load affect how sick I will get?

The relationship between viral load and severity of illness is complex – many different factors contribute, such as a patient’s age, pre-existing conditions, immune health, vaccination status, and available medical care.

In other diseases, such as influenza or other coronaviruses like SARS and MERS, high viral load usually does increase the severity of the disease.

One study has shown that viral load of COVID-19 was up to 60 times higher in patients with severe symptoms compared to those infected with the virus but comparatively okay. But other studies suggest that even asymptomatic people had similar viral loads to symptomatic patients. Clearly, we are still learning about the specifics of viral load when it comes to COVID-19.

Is somebody with a high viral more load more infectious?

Generally, yes. The more viral particles in the body, the higher the chance of spread. This is called viral shedding, when infectious viral particles exit the body, usually through breathing, coughing, speaking or sneezing.

When is viral load the highest?

Viral load appears to be highest for COVID-19 in the early days of infection; it peaks just before or around the time symptoms first appear. This means that viral transmission to others often occurs before a carrier even knows they have been infected. Load levels usually drop five days after symptoms begin.

Compared to SARS, viral shedding of COVID-19 peaks a lot earlier, which means it is much harder to identify and contain the virus. This is one of the reasons COVID-19 spreads so quickly compared to some other diseases.

Is viral load the same if I am vaccinated?

A study led by the University of California, Davis and recently posted on pre-print server MedRxiv found no significant difference in viral load between vaccinated and unvaccinated people who tested positive for the Delta variant.

UC Davis news reported that, out of a total of 869 positive cases examined, the researchers found wide variation in viral load within both vaccinated and unvaccinated groups, but not between them. There was no significant difference in viral load between vaccinated and unvaccinated, or between asymptomatic and symptomatic groups.

Results such as these are interesting, but they need to be treated with caution.

“We know already that when breakthrough infection occurs, and actual viral load is measured, that you can have similar viral loads in breakthrough cases as you do with primary cases,” says vaccine expert Professor Robert Booy, from the University of Sydney.

“However, within days of infection, the viral load in vaccinated people drops much more rapidly and therefore makes them much less likely to transmit and much less likely to be hospitalised or die.

“At diagnosis, there may be a similar load, but there are dynamic occurrences after that point which lead to vaccinated people having fewer symptoms and being less likely to transmit.”

Ultimately, that can mean a vaccinated person is at peak viral load for significantly less time than an unvaccinated person, and so infects fewer people. Beyond that, vaccines have been shown to reduce symptoms and hospitalisation.

The second consideration is that transmission is complex. Breakthrough infections are uncommon, so fewer people in a highly vaccinated population are getting sick or infectious.

On an individual level, one single vaccinated and infected person may have the same viral load at one time point as an unvaccinated and infected person – but the big difference is that the chance of getting sick is significantly reduced. At a population level, that translates to vaccines being highly effective at reducing spread and hospitalisation.