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.
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https://www.nytimes.com/interactive/2021/03/03/science/vaccine-efficacy-...
