Why Are Vaccinated People Getting Covid-19?

Base rates, denominators, and some really basic epidemiological maths

Gideon M-K; Health Nerd
6 min readJul 28, 2021


Pictured: Life-saving. Get your Covid-19 vaccine now! Source: SELF Magazine

As the world slowly limps forward from the battering of a global pandemic into a post-vaccine world, there’s a common, and very understandable question being asked across the globe. Why, if vaccines work, are we seeing more and more people getting sick after being immunized against Covid-19? Shouldn’t we all be protected now that we’ve been jabbed in the arm?

Stock photo results for “protected” are filled with a surprising number of umbrellas. Oddly, not many pics of condoms. Source: Pexels

And if you look at the figures, it’s absolutely true — in many places where lots of people are vaccinated, most coronavirus infections are among people who have been immunized. This seems to contradict the well-demonstrated fact that vaccines prevent up to 95% of symptomatic disease, and are incredibly effective at stopping you from getting Covid-19.

What’s going on here? Allow me to introduce you to a simple cognitive twist called the base rate fallacy, and go back to the First Commandment of Epidemiology: Thou Shalt Always Remember The Denominator.

Pictured: a very silly graphic I made at the start of the pandemic

Base Rate Fallacy

What is the base rate fallacy? To put it simply, when we’re looking at proportions, we tend to forget the base rate of something and instead focus on a specific part of the equation that seems most pertinent to our lives.

A really commonly-used example of this is seatbelts and automobile accidents. If you look at the statistics, the majority of people who die in car crashes were wearing seatbelts. Usually it’s well over 50%. Now, in isolation, that sounds quite scary — it sounds like you’re more likely to die if you’re wearing a seatbelt — but once you look at the context this actually proves just how amazing seatbelts are.

Seatbelts have saved more lives than we can easily comprehend. Source: Pexels



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