Birx: “Key metric that I watch is the positivity rate”

In yesterday’s post, I indicated that I was very disappointed with Dr. Brix’s answers  in a TV interview with Bret Baer.

One particular question & answer still has me scratching my head:

What is the key statistic that you track?

Birx’s answer: Test positivity (i.e. ratio of positive test results to total tests). It’s the most sensitive indicator and best early warning.

With all due respect, I think that Dr. Birx is confusing “positivity” with “prevalence”.

I’m way more interested in the latter prevalence: the percentage of the population that is currently infected with the virus.

Prevalence indicates how widespread the virus is at any point in time in a selected locale.

That gives me a sense of how safe it is to leave my house: How many people am I likely to run into who have the virus and may be contagious.


Determining prevalence requires periodic random sampling of the population.

That’s not what’s being done now.

A representative sample of the local populations is not being tested.

Why is that a problem?

The positivity rate (Birx’s key metric) is a function of who shows up to be tested.

If only people with covid-like symptoms are being tested, then of course, the positivity rate will be high.

If there’s a groundswell of asymptomatic people, the positivity rate will likely be relatively low.

My hunch: The testing “sample” is skewed to people with symptoms.

In a prior post, we reported that “Asymptomatics” are not rushing to get tested and provided some subjective reasons why that might be (e.g. no doctor referral, high hassle factor, privacy concerns).

So, positivity is, at best, a very crude measure of prevalence.


To that point, keep in mind…


On average, the positivity rate has been running under 10%

That means that over 90% of the people being tested are getting negative results.

What does that tell us?

My view: It sets the upper bound on the current prevalence rate … which is likely to be less than 10% … maybe way less.


Also keep in mind that  — of those testing positive — only some are serious cases that require  hospitalization.

It’s likely that most are test-positives are just instructed to stay home.

Wouldn’t it be nice to know the mix of people being tested … the treatment that they’re being prescribed … and, their outcomes?

And, oh yeah, wouldn’t it be nice to know the current covid prevalence rate in your community?


Later in the interview, Dr. Brix offered :

“We’re launching a study to determine where the virus has been, who has been infected, who is susceptible to infection and who transmits it.”

In other words, 6 months into the pandemic, “the science” still can’t answer the fundamental questions.

That makes it a lot harder to follow the science…


P.S. It’s still my view that daily new deaths is the key metric to track.


3 Responses to “Birx: “Key metric that I watch is the positivity rate””

  1. Deepak Gupta Says:

    Prof. Homa,

    I understand and agree with your opinion about experts and to an extent agree with your criticism of them. I know in past you have put the onus of President’s bungled up response on incorrect data provided by experts, but irrespective, a CEO/leader is who is responsible for the final outcome, so to me he can’t get away with the responsibility.
    Experts consistently said wearing Masks saves lives, don’t you think we should not done politics around Masks. We could have avoided losing so many lives ( a personal friend passed away yesterday, so I am feeling the pain). When Masks were in short supply Trump made a fanfare visit to Honeywell N 95 Mask factory. The factory had to throw the entire production that day because he didn’t wanted to wear a Mask. I feel very frustrated when I see politics in basic matters which are fundamental as such as wearing a Mask.

  2. Alejandro Says:

    Gupta, your credibility suffers when you state “Experts consistently said wearing masks saves lives…”


    March 8, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, says: “There’s no reason to be walking around with a mask.” (As we know, he later reversed position.)

    On April 3, the Centers for Disease Control and Prevention reversed its earlier position on the use of face masks during the COVID-19 pandemic and announced that it was now recommending that people should wear face coverings in public.

  3. How many people are walking around infected with Covid-19? | The Homa Files Says:

    […] Current testing procedures don’t answer the question (see: Why is COVID testing still so haphazard?) … so, the “experts” fixate on metrics that are muddled and potentially misleading (see: Birx: “Key metric that I watch is the positivity rate”). […]

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