2 companies say their vaccines are 95% effective. What does that mean?
NEW YORK, Nov 21: The front-runners in the vaccine race seem to be working far better than anyone expected. Pfizer and BioNTech announced this week that their vaccine had an efficacy rate of 95%. Moderna put the figure for its vaccine at 94.5%. In Russia, the makers of the Sputnik vaccine claimed their efficacy rate was over 90%.
"These are game changers," said Dr. Gregory Poland, a vaccine researcher at the Mayo Clinic. "We were all expecting 50 to 70%." Indeed, the Food and Drug Administration had said it would consider granting emergency approval for vaccines that showed just 50% efficacy.
From the headlines, you might well assume that these vaccines - which some people may receive in a matter of weeks - will protect 95 out of 100 people who get them. But that's not actually what the trials have shown. Exactly how the vaccines perform out in the real world will depend on a lot of factors we just don't have answers to yet - such as whether vaccinated people can get asymptomatic infections and how many people will get vaccinated.
Here's what you need to know about the actual effectiveness of these vaccines.
What do the companies mean when they say their vaccines are 95% effective?
The fundamental logic behind today's vaccine trials was worked out by statisticians more than a century ago. Researchers vaccinate some people and give a placebo to others. They then wait for participants to get sick and look at how many of the illnesses came from each group.
In the case of Pfizer, for example, the company recruited 43,661 volunteers and waited for 170 people to come down with symptoms of COVID-19 and then get a positive test. Out of these 170, 162 had received a placebo shot, and just eight had received the real vaccine.
From these numbers, Pfizer's researchers calculated the fraction of volunteers in each group who got sick. Both fractions were small, but the fraction of unvaccinated volunteers who got sick was much bigger than the fraction of vaccinated ones. The scientists then determined the relative difference between those two fractions. Scientists express that difference with a value they call efficacy. If there's no difference between the vaccine and placebo groups, the efficacy is zero. If none of the sick people had been vaccinated, the efficacy is 100%.
A 95% efficacy is certainly compelling evidence that a vaccine works well. But that number doesn't tell you what your chances are of becoming sick if you get vaccinated. And on its own, it also doesn't say how well the vaccine will bring down COVID-19 across the United States.
What's the difference between efficacy and effectiveness?
Efficacy and effectiveness are related to each other, but they're not the same thing. And vaccine experts say it's crucial not to mix them up. Efficacy is just a measurement made during a clinical trial. "Effectiveness is how well the vaccine works out in the real world," said Naor Bar-Zeev, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
It's possible that the effectiveness of coronavirus vaccines will match their impressive efficacy in clinical trials. But if previous vaccines are any guide, effectiveness may prove somewhat lower.
The mismatch comes about because the people who join clinical trials are not a perfect reflection of the population at large. Out in the real world, people may have a host of chronic health problems that could interfere with a vaccine's protection, for example.
The Centers for Disease Control and Prevention has a long history of following the effectiveness of vaccines after they are approved. On Thursday, the agency posted information on its website about its plans to study the effectiveness of coronavirus vaccines. It will find opportunities to compare the health of vaccinated people to others in their communities who have not received a vaccine.
-The New York Times.