Food for thought: Karl Popper and studies of Covid vaccines

Eyal Shahar
3 min readApr 16, 2023


Karl Popper, a 20th century philosopher of science, is not a household name, and many philosophers, statisticians, and epidemiologists disagree with his views. But even his opponents probably agree with two of his central ideas:

  • No scientific theory can ever reach the status of certain truth.
  • Deductive reasoning plays an important role in science.

Popper argued that scientific theories are universal claims about reality, unconstrained by place and time. Since it is impossible to verify that a universal claim holds in every set of circumstances, it is impossible to show that a scientific theory is true, no matter how many times it has been corroborated. It is possible to show, however, that a theory is false, using deductive reasoning.

The classical example in philosophy was mentioned in a recent Brownstone article on Covid vaccines:

“…in scientific discourse, a single ‘black swan’ as termed by Karl Popper — a single negative instance that does not fit in with the theory — may falsify a universal claim;”

Deductive reasoning tells us that “All swans are white”, a universal claim, must be false if we observe a black swan. Simple inference.

That does not mean, of course, that falsification is ever certain. Did the observer report the truth? Was it a swan or something that looked like a swan? Perhaps it was a white swan covered with black dirt. Every observation is compatible with more than one explanatory theory. That’s another key idea of Popper and others.

Neither a scientific theory, nor any observation, nor many observations can ever attain the status of certain truth, though many scientists are satisfied with reaching a psychological state of feeling certain or believing strongly. There is even a school of statistics, called Bayesian statistics, that is concerned with updating beliefs on the basis of new data.

Whether you are Popperian, Bayesian, or something else, one idea should be accepted by anyone who subscribes to logic: No statement about causal reality is ever logically certain. You cannot establish in science anything like the (axiom-based) logical certainty of a theorem in math. Therefore, as far as scientific knowledge is concerned, there cannot be authorities. There are no individuals, groups, or institutions who know the truth. “Follow the science” is an empty slogan. The science does not speak, and no one should claim to speak for “the science”.

Popper eloquently summarized:

“There are all kinds of sources of our knowledge; but none has authority… The fundamental mistake made by the philosophical theory of the ultimate sources of our knowledge is that it does not distinguish clearly enough between questions of origin and questions of validity.”

The Centers for Disease Control and Prevention in the US (an origin of knowledge) may tell us that Covid vaccines are highly effective against Covid death, but that does not mean that the statement is true (valid knowledge).

I stopped counting how many studies and dashboards have corroborated the official narrative of “highly effective Covid vaccines”. Back in 2021, one observational study after another was delivering outstanding results, reporting 90% vaccine effectiveness or higher, much better than flu vaccines.

Too good to be true?

What should skeptical minds do? What would Popper advise? Do we need to try to show the pitfalls of every study?

Not necessarily.

First, we may point out one key bias — healthy vaccinee bias — that is shared by studies of vaccine effectiveness and is not easily rectified by conventional methods. Demonstrated numerous times in the case of Covid vaccines [here, here, here, and here], the bias would lead to over-estimation of vaccine effectiveness.

Second, we may find a study that does not agree with all others. If one study informs us (correctly, of course) that the effectiveness of Covid vaccines is far from 90%, then every study that reported 90% effectiveness must be at fault (deductive reasoning).

Make no mistake, we cannot show anything with logical certainty. There might not even be a single number to quantify effectiveness (due to effect modifiers). But we can try to show why the prevailing, sweeping claim — highly effective — is grossly mistaken. That’s the essence of the critical method of science.

If there is any meaning to “follow the science” it is this: Follow scientific claims and their critiques — as critically as you can.



Eyal Shahar

Professor Emeritus of Public Health (University of Arizona); MD (Tel-Aviv University, Israel); MPH, Epidemiology (University of Minnesota)