More on the COVID vaccines’ 95% effectiveness rate…

Probably overstated but, nonetheless, I’ll get vaccinated as soon as I can!

It’s undeniable that Operation Warp Speed’s financial support and streamlined regulatory processes motivated rapid development of COVID vaccines,

That was largely predictable.

What wasn’t so predictable was the apparent sky high effectiveness of the early-launch vaccines.

Both Pfizer and Moderna report about 95% effectiveness.

Gotta ask: Are these effectiveness rates too good to be true?

In a prior post, we noted that  the 95% effectiveness is, indeed, sky high compared to previous flu and pandemic virus vaccines.

Today, let’s drill down on the 95% number…


First, some basics…

According to the WSJ:

44,000 adults in the U.S. and in other countries took part in Pfizer’s study,

Half got the vaccine and half got a placebo.

None of the participants were injected with the virus; all were simply returned to their “everyday life” environments and regularly monitored.

[The study was considered “complete” when a target number of infections were observed.]

Of the 44,000, 170 developed Covid-19 with at least one symptom.

Out of those, just 8  had taken the vaccine, while 162  had received a placebo.

So, how to get to the 95% number?

Restating the result: the vaccinated group had 154 fewer symptomatic infections  than the placebo group … so the implied effectiveness rate is 95% (154 / 162 = 95%).


Practically speaking, what does the 95% really mean?

A common misconception is that study participants were given the vaccine, then injected with the virus … and then either fought it off or developed symptoms.

That’s not the case,

For ethical reasons, studies are rarely done that way.

Doing so would put the placebo group at a level of health risk that is generally considered morally unacceptable.

All participants simply went on with their usual lifestyle routines.

Some of the participants may have had COVID anti-bodies already in their system (so called T-cells or remnants of asymptomatic COVID infections).

That’s a partial explanation for why the overall infection rate (.4%) is so low [170 / 44,000 = .4%]

Also, assuming participants were following CDC guidelines (i.e. minimizing public contacts, socially distancing), many probably had few (or no) contacts with COVID infected transmitters.

Bottom line: It would be a stretch to conclude that, for example, there’s only a 5% chance of getting infected if you’re substantially exposed to the virus by close contact with a contagious person.

The exact number — a more stringent measure of effectiveness — isn’t known based on the study … but, it is certainly higher than 5% … which means that the “real” effectiveness rate is less than 95%


Also, note the fine print:

Of the 44,000 participants, 170 developed Covid-19 with at least one symptom.

Said differently, the study only counted symptomatic infectees.

The study either didn’t test & identify asymptomatic infectees … or just didn’t report the number of participants who got infected but didn’t exhibit symptoms.

Recent estimates are that there are as many as 5 asymptomatic infectees for every one who is symptomatic.

If that number is correct, then the vaccines’ effectiveness — defined as fending off an infection, not just a symptomatic infection — drops down to about 70%.

That’s still very high by historical standards … just not as incredibly high as current headlines might lead one to conclude.

In a prior post, we noted that  95% is sky high compared to previous flu and pandemic virus vaccines.


Bottom line: Going into this situation, a 70% effectiveness rate would have been considered a monumental achievement.  High enough that I’d be rushing to get vaccinated.

So, even if the 95% is a bit overstated, I’m all in.

One Response to “More on the COVID vaccines’ 95% effectiveness rate…”

  1. Still more on the COVID vaccines’ 95% effectiveness rate… | The Homa Files Says:

    […] a prior post, we parsed the Pfizer study results … honing in on one of the study’s limitations: it only […]

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