colorized human cells (pink) heavily infected with SARS-COV-2 virus particles (green) / NIAID

Viral Load as a Predictor of COVID-19 Patient Outcomes

A Q&A with Daniel Griffin

Back in early March when most of the world was learning about COVID-19 for the first time, Daniel Griffin, MD, PhD, clinical instructor of medicine, and Stephen Goff, PhD, the Higgins Professor of Microbiology & Immunology and Biochemistry & Molecular Biophysics, were already examining early data from France and noticing a distinct pattern around viral load, the amount of virus detected in a PCR nasal swab, and patient outcomes. They saw that the amount of virus detected in COVID-19 patients experiencing mild symptoms would drop significantly within one week of infection, whereas patients who still had high levels of virus detected after a week of infection were significantly more likely to be hospitalized or die.  

How can clinicians use viral load to improve survival from COVID-19?

You first have to determine what stage of disease the patient is in at the time viral load is measured via a PCR test. If you have a patient whose viral load has not dropped within a week of first experiencing symptoms, you can expect that this patient is going to need hospitalization or other interventions such as breathing support. You should also understand that this patient can still transmit the disease to others, so quarantining away from family members is important. In terms of treatment, these patients should seek monoclonal antibody infusion as soon as possible. 

The flip side of this is that some patients will continue to test positive for COVID-19 via a PCR test but have a very low viral load. These patients do not need to stay hospitalized or isolated as their bodies have successfully fought off the virus and they are no longer contagious to others.

Why have testing companies been slow to report viral load results?

As we know all too well, it took a long time early on to even get testing for COVID-19 available. When tests were finally available, the information on viral load was always there, meaning the machines were indeed measuring it. But the FDA did not approve the tests for quantitative results, which is how viral load is categorized. It wasn’t until earlier this month (December) that the FDA allowed this information to be shared with clinicians. I don’t know for sure why they reversed course, but it’s possible it was a result of the published literature from research centers that persuaded them of the value of viral load. It’s also possible that the attention given to President Trump’s viral load led to greater awareness that this is indeed a key marker when discussing recovery and contagion. 

You've recently published on how schools and business should implement testing protocols. In a scenario where one student or worker tests positive, should viral load be taken into account for testing and quarantining of others who were exposed?

Ideally in this scenario the infected individual gets two tests and we can assume that the first one was taken as a result of experiencing symptoms. The viral load level of the first test really matters; if this number is high, we could conclude the individual is very contagious and therefore all close contacts should be tested and quarantined. 

Alternatively, if the viral load is low at the first test, he or she may be at the tail end of disease and no longer contagious. 

In an ideal world we would be screening people regularly and picking up the virus before someone is experiencing symptoms or infectious. That’s really the gold standard in terms of how testing and viral load can be used to prevent new cases, but we are not there yet.