Covid Vaccine Efficacy - Visualising Simpson’s Law

less than 1 minute read

I recently came across a tweet which mentioned the likelihood that recent Covid vaccine efficacy stats coming out of Israel were an example of a famous statistical paradox generally attributed to the British statistician Edward Simpson but noticed by a number of predecessors (see the Wikipedia entry for details. Here’s the tweet in question:

Bak-Coleman tweet

The paradox is that while the efficacy (this term is explained in the linked visualisation) of the vaccine when assessed against the whole population of unvaccinated and fully vaccinated (67.5%) looks low, if we separate the population into over and under 50 years old the efficacy for each of those groups is considerably higher at 91.8% for under 50s and 85.2% for over 50s.

Now this result is pretty counterintuitive for most people but, as the above example shows, failure to grasp it could have serious consequences for health policy . I thought that a little data visualisation might help, showing a smooth transition between a population of all ages and two subpopulations, filtered by age (> and < than 50) and the resulting effect on vaccine efficacy.

Check out the visualisation here