If you look at Covid data from Israel across all ages, vaccine efficacy against severe disease is 67.5%. But if you break it down by age it turns out to be significantly higher: for those under 50 it’s 91.8%, and those over 50 it’s 85.2%. What’s going on? “Simpson’s paradox arises when there are ‘lurking variables’ that split data into multiple separate distributions.”
In this case: “… since older people are more likely to get sick with Covid but also more likely to be vaccinated, looking at all ages instead of breaking out specific age groups can greatly understate how effective the vaccines really are.”