Your questions, answered
“With respect to the vaccines, could you explain/define what 90 or 95 percent effective really means?” — Leeann in Maryland
We’ve been getting this question a lot in the past two weeks, no doubt because of all the headlines announcing that two U.S. vaccine candidates, Pfizer/BioNTech’s and Moderna’s, each proved more than 90 percent effective in their preliminary trials.
Before vaccine developers can submit their products to the government for approval, they are required to conduct studies with many thousands of volunteers. About half of those volunteers get the real vaccine, and half get an inert placebo. Several weeks or months pass, and the researchers then count how many people in each group got sick during that time.
Per the Centers for Disease Control and Prevention, a vaccine’s efficacy simply measures “the proportionate reduction in cases among vaccinated persons,” as compared to the group that got a placebo in the same study.
For example, Pfizer/BioNTech announced last month that out of more than 43,000 volunteers in their study, 170 people contracted covid-19. Of those 170, only eight people had received the real vaccine, and 162 had taken a placebo. Thus, the vaccine was estimated to be about 95 percent effective. In other words, the rate of disease was 95 percent lower among the vaccinated group than among the placebo group.
Moderna announced very similar results Monday: 196 volunteers in their study contracted covid-19, “of which 185 cases of covid-19 were observed in the placebo group, versus 11 cases” in the vaccinated group. That worked out to a vaccine efficacy of 94.1 percent. (If you're trying to replicate the math at home, be warned that it's more complex than simply dividing 185 by 196, but you'll be in the general ballpark.)
These results appear to be very, very strong. By comparison, seasonal flu vaccines typically have efficacy percentages in the 60s or lower. But it's important to remember they are merely measuring the results of controlled clinical studies. We won't know how well the vaccines work in the real world until and unless they are approved by the government and administered to tens of millions of Americans.
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