One question about coronavirus which gets a lot of attention is about ‘viral load’.
But as well as there often being confusion over terminology here, there is very little published research specific to SARS-CoV-2 at this point – so expert views mostly consist of inferrals made from what we know about other viruses. Let’s take a look.
Viral load refers to how much virus is present in an infected person’s body. With other viruses that we understand better – the higher the viral load, the more severe the disease.
Viral load is also associated with ‘shedding’, which describes how much virus an infected person is producing and releasing, which could potentially infect others.
Viral load is often confused, however, with something called the ‘minimal infective dose’. This refers to the smallest number of virus particles needed to infect 50% of individuals. It is a statistical average therefore, which means the actual minimum number of virus particles required to infect could vary slightly between individuals.
There is no specific research yet on the minimal infective dose for Covid-19. But experts have pointed to observations that the virus seems to be highly infective with only very little contact, which would suggest a low minimal infective dose.
Also, other similar viruses such as SARS have very small minimal doses, so for this coronavirus it could be that just a few hundred particles are needed to infect someone.
One study published earlier this month in the journal Nature did find that patients with only mild symptoms still had a relatively high viral load in their throats, which could explain why the virus is so infectious through coughing and sneezing.
The study was only in nine patients, however, which in scientific terms is very small. Experts commenting on the findings said the results were not conclusive, but may help in understanding how coronavirus has spread so quickly.
Away from the theory, clinicians are assuming that a patient with severe COVID-19 would have a high viral load (producing lots of virus particles), and could be the source of a high infective dose for someone else.
This is one reason to prioritise personal protective equipment for medical workers, as this is where the highest risk of severe infections lies.