COVID Questions: Facts About Effectiveness of Testing

Creative rendition of SARS-COV-2 virus particles. Credit: National Institute of Allergy and Infectious Diseases, NIH
Creative rendition of SARS-COV-2 virus particles. Note: not to scale. (Credit: National Institute of Allergy and Infectious Diseases, NIH)

Note: UW-Madison will be publishing answers to questions about COVID-19 and the pandemic each week in a COVID questions column. If you have a question, please email it to covid19update@uc.wisc.edu. This post was originally published here.

Q: A lot of people have been talking about how new variants of SARS-CoV-2 may cause our existing vaccines to be less effective. How much would the virus need to mutate before we became concerned about the effectiveness of our testing?

A: There are many different tests for SARS-CoV-2. Rapid antigen tests detect viral proteins, components of the viruses themselves that may be in people’s saliva or nasal secretions. These tests use antibodies that bind to virus proteins. A random mutation in the virus that changed the regions of viral proteins that these antibodies attach to could potentially affect the sensitivity of such tests, but they are generally designed to recognize parts of the virus that do not seem to tolerate much mutation.

Most other tests use polymerase chain reaction (PCR) to detect the genetic material of the virus. Many, but not all, PCR tests look for several different parts of the virus’s genetic material. This means that even if a mutation prevented detection of one test target, other targets would still be detected and the test would be called positive. In fact, this is one way that people noticed the emergence of the variant B.1.1.7 in the UK — one of the mutations carried by that virus causes one of three virus-specific reactions in a particular test to fail, giving a characteristic pattern that suggests the presence of the mutant.

So, in theory, one mutation may be enough for certain tests to fail, but it would have to randomly be in just the right region of the virus to prevent detection in a given test. The fact that so many different tests are being used means that no one mutation will cause the virus to be undetectable across the board. People are increasingly looking at mutations that develop to determine whether they might affect the sensitivity of various tests.

—Thomas Friedrich, Professor, Pathobiological Sciences, UW School of Veterinary Medicine; Virology Services unit head, Wisconsin National Primate Research Center


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