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. In addition, 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: For statisticians, efficacy is a measurement of how much a vaccine lowers the risk of an outcome. For example, Johnson & Johnson observed how many people who received a vaccine nevertheless got Covid-19. Then they compared that to how many people contracted Covid-19 after receiving a placebo.
The difference in risk can be calculated as a percentage. Zero percent means that vaccinated people are at as much risk as people who got the placebo. A hundred percent means that the risk was entirely eliminated by the vaccine. In the United States trial site, Johnson & Johnson determined that the efficacy is 72 percent.
Efficacy can also change when scientists look at different outcomes. Johnson & Johnson’s vaccine had an 85 percent efficacy rate against severe cases of Covid-19, for example. That’s important to know, because it means that the vaccine will prevent a lot of hospitalizations and deaths.
For a number of reasons, it’s not possible to make a precise comparison between vaccines. One vaccine may have a higher estimate than another, but their confidence intervals (the range of possible results) may overlap. That effectively makes their results indistinguishable.
Making matters more complicated, the vaccines were tested on different groups of people at different stages in the pandemic. In addition, their efficacy was measured in different ways. Johnson & Johnson’s efficacy was measured 28 days after a single dose, for example, while Moderna’s was measured 14 days after a second dose.
What’s clear is that all three vaccines made by Johnson & Johnson, Moderna, and Pfizer and BioNTech — greatly reduce the risk of getting Covid-19 and as if they have a high efficacy against more serious outcomes like hospitalization and death.
Once a vaccine goes into widespread use, researchers follow its performance. Instead of efficacy, these scientists now measure effectiveness: how much the vaccine reduces the risk of a disease out in the real world, in millions of people rather than the thousands that were in the studies. Early studies on the effectiveness of coronavirus vaccines are confirming that they provide strong protection.
In the months to come, researchers will keep an eye on this data to see if they become less effective — either because the immunity from the vaccine wanes or because a new variant arises. In either case, new vaccines will be created, and manufacturers will provide new measures of their efficacy.
Jackie
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