What a vaccine effectiveness figure does not tell your patient

Vaccine effectiveness is a relative measure, and patients hear it as an absolute one. Told a vaccine is 80% effective, most people understand that eight in ten vaccinated people will be protected from something that would otherwise have happened to them. What it actually means is that among vaccinated people, the outcome occurred about a fifth as often as among unvaccinated people — and how much that is worth depends entirely on how likely the outcome was in the first place.

So pair every percentage with a baseline. The same 80% figure means something very different to a healthy 30-year-old and to an 80-year-old with heart failure, and the second patient gains far more from the same number. This is also why effectiveness against hospitalisation and effectiveness against infection are not interchangeable, and why quoting the wrong one erodes trust when the patient later catches the illness.

Name the outcome, name the population, name the baseline. Three extra seconds, and the number stops being a slogan.

• State the outcome the figure refers to — infection, hospitalisation or death are different numbers
• Pair the percentage with this patient's baseline risk, not a population average
• Say plainly that vaccination reduces severity more reliably than it prevents infection
• Give the interval, or at least say how certain the figure is, when it is wide
• Note when an estimate comes from a different population or product than the one you are offering

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