Explainer: Some key terms in vaccine efficacy

 This is an excerpt from my blog posts on the Covid 19 vaccine race at Absolutely Maybe.

  • Homologous vaccine schedules have the same vaccine for each dose. When the doses aren’t all of the same vaccine, that’s called heterologous – which I call “mixed” in my posts. The first shot in a multi-dose vaccine schedule is called the prime: doses thereafter are boosts.
  • Vaccine efficacy is a rate of risk reduction in symptomatic Covid-19 unless otherwise specified. Vaccine efficacy of 80% or 90% means if a vaccinated person is exposed to the virus, their risk of getting the disease is lowered by that proportion, so it depends on how high their risk of being exposed, and that varies. It’s not an absolute drop in percentage points. (Efficacy is for results on disease outcomes from phase 3 clinical trials; effectiveness studies follow that.)
  • When available, a range of statistical certainty for efficacy is shown, eg 92% (CI: 88-95). The distance between 88% and 95% in this example is small: it means there is a lot of certainty that 92% is about what we can expect. However, the wider that range is, the more uncertain we are.
  • The rate of efficacy set for whether a Covid vaccine works well enough is 50% (with a CI starting at 30% at least).
  • And I have a post explaining the terms used, and assessment processes, for adverse events and safety in these trials – including the difference between “severe” and “serious”.
Hilda Bastian
Posted 20 April 2023

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