Explanation of vaccine efficiency--NY Times

What Do Vaccine Efficacy Numbers Actually Mean? All of the F.D.A.-authorized vaccines offer strong protection against Covid-19, and assessing their efficacy isn’t as simple as a head-to-head comparison. nytimes

This week, Johnson & Johnson began delivering millions of doses of its coronavirus vaccine across the United States after receiving an emergency use authorization from the Food and Drug Administration. Central to getting the green light was a trial that Johnson & Johnson ran to measure the vaccine’s efficacy.

Efficacy is a crucial concept in vaccine trials, but it’s also a tricky one. If a vaccine has an efficacy of, say, 95 percent, that doesn’t mean that 5 percent of people who receive that vaccine will get Covid-19. And just because one vaccine ends up with a higher efficacy estimate than another in trials doesn’t necessarily mean it’s superior. Here’s why.

...  see link for graphs and detailed explanation