Expertise
Vaccines
How vaccines teach the immune system, what efficacy figures do and do not mean, and how protection is measured in the real world.
Described in words
A bar showing the protection measured under controlled trial conditions, next to a shorter bar showing protection measured in everyday use, with the practical reasons for the difference listed between them.
A vaccine is a rehearsal. It presents the immune system with a recognisable feature of a pathogen under conditions that are not dangerous, so that a real encounter is met with a prepared response instead of an improvised one.
Most public confusion about vaccines is not about that idea. It is about the numbers attached to it — what a 90% figure counts, who it was measured in, and what changes when the same product is used in a whole population rather than a trial.