COVID questions: Viral load, the value of life

Editor’s note: We will be publishing answers to questions about COVID-19 and the pandemic each week in this COVID questions column. If you have a question, please email it to covid19update@uc.wisc.edu.

Q: A recent poll asked Americans if the current number of deaths from COVID-19 was acceptable or unacceptable, with the answer varying widely. Our government routinely makes laws and regulations weighing the risk of death against economic costs. How are those decisions typically made, and how are they being applied in our current situation?

A: Federal agencies are required to conduct cost-benefit analyses of major proposed rules as components of Regulatory Impact Analyses. Rules often reduce mortality risk. For example, a rule requiring some sort of safety device on automobiles might be predicted to reduce mortality risk from highway accidents.

To monetize the benefit, the analysts would predict the number of deaths avoided and multiply them by an estimate of the value of a statistical life (VSL), which is an estimate of how much on average people implicitly value their own lives in making decisions involving tradeoffs between small changes in mortality risk and things of value such as money or time.

Most federal agencies currently use values of about $10 million for the VSL, which is an average value for the U.S. population. (Note that the VSL is not what an individual would pay to avoid death, but rather what implicit value the individual is putting on his or her own life in making decisions involving small changes in mortality risk, such as employment choices, purchases of safety equipment, or participation in enjoyable but risky activities.)

In the case of avoided highway deaths, the Department of Transportation currently uses a value of $9.6 million for the VSL. Thus, a federal rule that would have avoided 100,000 pandemic deaths in the United States would have been monetized at about $1 trillion. Of course, cost-benefit analysis only assesses efficiency. A fuller analysis would use the estimated net benefits from the cost-benefit analysis as a measure of efficiency that would be weighed against other social values such as equity.

– David L. Weimer, Edwin E. Witte Professor of Political Economy, Robert M. La Follette School of Public Affairs

Q: What is a viral load? How is it used to test for COVID?

A: Viral load is a term we use to describe the amount of virus present in a sample. We typically use this term to mean the amount of virus detected in tests that look for viral genetic material. The most commonly used test for SARS-CoV-2 infection right now uses a technique called PCR to specifically detect the genetic material (RNA) of the virus.

Although the results given to clinicians and patients are almost always just “positive” or “negative,” behind the scenes these PCR tests can do more than just detect the presence of viral RNA. The results of the tests also let us estimate how much viral RNA was in the patient’s sample. We can use this as a proxy for how many viruses were in the patient’s sample, and that number is what we call the viral load.

This is important because other studies have shown that a person’s viral load can be used to determine whether or not they are likely to be contagious to others. Viral load can have a huge range of values, from tens of RNA genomes to hundreds of billions in one milliliter of fluid.

There is a viral load threshold below which scientists usually cannot detect *infectious* SARS-CoV-2 in a sample — there’s enough viral RNA there to be detected in very sensitive PCR tests, but not enough live viruses to pose a high risk of spreading infection to someone else. The exact threshold has been estimated somewhat differently in different studies, and probably depends on the types of test used, types of samples collected and other things, but it is probably between 1 million and 10 million copies per milliliter. Sometimes these results are reported as “threshold cycle,” or CT, which is somewhat less precise but still informative. In CT terms, a lower number means more virus. Infectious virus is rarely detected in samples with CTs greater than 30-33.

So, in sum, the viral load tells us how much virus is present in a person, in terms of viral genetic material. It is very informative! In my view, viral load information should be included with SARS-CoV-2 PCR test results—we should receive a value, not just “positive” or “negative.”

– Thomas Friedrich, Professor, Pathobiological Sciences; Virology Services unit head, Wisconsin National Primate Research Center

See more answers to COVID questions at https://news.wisc.edu/tag/covid-questions/. Also, visit our COVID-19 impact site.