I am... perplexed by this quote from Paul Offit in this piece by @taraparkerpope.

95% vaccine efficacy doesn’t mean that “95% of people are protected.”

https://www.nytimes.com/2021/04/02/well/live/covid-vaccine-side-effects-faq.html

95% efficacy means the odds of a vaccinated person getting sick *after a given exposure* are 95% lower than unvaccinated person.

But *repeated* exposures may well breakthrough vaccine protection in some share of vaccinated people. The trials weren’t designed to test this.

Not every person in the trial was challenged by COVID. In the placebo group, only ~2% of people contracted COVID.

So we can’t say that 95% of vaccinated people are simply “protected.” We don’t know what % are susceptible to breakthrough infection, or how many exposures it takes.

This is why herd immunity is important: the best protection is never getting exposed at all.

If enough people are vaccinated, we’re like a wet forest: a spark ( a few cases) can’t spread and set off an outbreak. But a raging wildfire (epidemic) can burn through even damp wood.

Tl;dr — 95% efficacy doesn’t mean a 95% chance you’re impervious to COVID.

Even a highly effective vaccinated can be overwhelmed if population immunity is low enough that the epidemic runs wild, and vaccinated people are constantly taking shots on goal.

Here is a simple herd immunity simulator that shows what happens with a 90% effective vaccine (cuts infection risk from 0.5 to 0.05) but low public vaccination rates (just 40%).

>20% of vaccinated people got infected, even with a 90% effective vaccine. https://www.software3d.com/Home/Vax/Immunity.php