In the US, the first two available Covid-19 vaccines were the ones from Pfizer/BioNTech and Moderna. Both vaccines have very high “efficacy rates,” of around 95%. But the third vaccine introduced in the US, from Johnson & Johnson, has a considerably lower efficacy rate: just 66%. Look at those numbers next to each other, and it’s natural to conclude that one of them is considerably worse.
Why settle for 66% when you can have 95%? But that isn’t the right way to understand a vaccine’s efficacy rate, or even to understand what a vaccine does. And public health experts say that if you really want to know which vaccine is the best one, efficacy isn’t actually the most important number at all.