False Negatives

As you hopefully know, the virus that’s been reaping havoc around the planet since December is SARS-CoV-2. That’s the name of the virus that causes the Covid-19 disease.

Status on 30 May 2020

One of the tests commonly in use to detect infection with this virus is the reverse transcriptase polymerase chain reaction test (RT-PCR). This test uses a person’s respiratory sample to detect viral particles and determine if the person may have been exposed to the SARS-CoV-2 virus.

Unfortunately, a report from Johns Hopkins this week tells us the following about the accuracy of this test. This report focuses on “false negative” results — reporting that a person is not infected when in fact he or she has the virus.

  • If tested on the day of infection, the test will fail to detect the virus in people who are infected, period. Negative results are meaningless.
  • On the fourth day after infection, the test will fail to detect the virus in 67% of people who have it.
  • On the eighth day after infection, the test will fail to detect the virus in 20% of people who have it.
  • Overall, the test produces a false negative result 36% of the time it is used.

Since people are asymptomatic when initial infection occurs, we can almost never know how long after infection a test is being administered. Between 38% and 75% of people with the virus never show symptoms, and most of those are never tested in the US, due to lack of contact tracing and test kits.

What does this mean?

  1. Anybody could be infected. One forecaster, mentioned in a previous post, predicted that a majority of the US population with have the virus by fall of this year. How that will interact with flu season becomes an interesting question. If infected with both viruses, will that dramatically increase the risk of pneumonia, or make pneumonia more lethal? Get a flu shot!
  2. The lack of knowing who has the virus is why it is certain that the number of cases and deaths will increase as quarantines are lifted.
  3. Government data on test kits and the number of tests administered is silly. If people on average will consume 2 to 4 test kits, we have enough kits for perhaps 2% of the US population, and will not have an adequate number anytime in the foreseeable future.
  4. Optimism about a vaccine should not change your behavior now or probably for the rest of this year. The unknowns include: (1) Will we actually get one that works? (2) When will it be in full production? (3) How many people will take it?

Virus mutation is a roll of the dice. Some mutations are harmless. Some are dead ends that the virus with that mutation dies out. However, at some point, there is a probability that we will see a mutation that combines the transmittability of the current corona virus with the lethality of the original SARS virus from 2003 (11% kill rate). Imagine the US with 33 million people dead and bodies piled on the street. Unless we learn how to deal with this, we will see that.


  1. Lauren M. Kucirka, Stephen A. Lauer, Oliver Laeyendecker, Denali Boon, Justin Lessler. Variation in False-Negative Rate of Reverse Transcriptase Polymerase Chain Reaction–Based SARS-CoV-2 Tests by Time Since Exposure. Annals of Internal Medicine, 2020; DOI: 10.7326/M20-1495
  2. Benjamin D. Singer. COVID-19 and the next influenza season. Science Advances, May 29, 2020: eabd0086 DOI: 10.1126/sciadv.abd0086

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