What Does Vaccine Efficacy Really Mean?

In this week's edition of the Covid Q&A, we look at what vaccine efficacy really means.

In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis is picking one question sent in by readers and putting it to an expert in the field. This week's question comes to us from Lois in Atlanta. Lois wonders what goes into determining a vaccine's efficacy rate. Lois asks:

When the news reports that such-and-such vaccine is 85% effective, what does that actually mean?