How Effective is that Efficacy?
The real deal and the ideal deal
Now that we have two (count ’em, TWO!) vaccines¹ out there, you have undoubtedly heard or read the numbers 95 and 94.5 bandied about, as in, “The Phizer/BioNTech vaccine is 95% effective!” and “Moderna’s vaccine is 94.5% effective!” However, this is not entirely accurate. We should say, “The Phizer/BioNTech vaccine displays 95% efficacy,” and “Modern’s vaccine displays 94.5% efficacy. Efficacy refers to how well vaccines perform under ideal conditions, such as in phase III vaccine trials. These are randomized, double-blind, and controlled — the gold standard of medical testing. There are well defined placebo and treatment groups, participants are monitored and followed up on, etc. Effectiveness, on the other hand, refers to how well a vaccine performs out in the wide world. Concerning these two vaccines, we don’t know that yet. However, it is important to obtain efficacy data so that we can make an intelligent decision as to whether or not a vaccine will work well enough out in the world to justify the risks it may entail there.
Both effectiveness and efficacy are calculated similarly. You subtract the percent of people who got the vaccine and got sick from the percent of people who did not get the vaccine and got sick. Then you divide that answer by the percent of people in the latter group and voilà, you are ready for the evening news.
It is not hard to understand how much trickier it is to make such calculations in the wide world than in a controlled study. But the wide world is where you have to go to test effectiveness. In that wide world you have to do observational studies. These are ongoing in the case of influenza vaccines, and they will no doubt be ongoing in the case of COVID-19 vaccines as well. In the case of observational studies you have to contend with the general hubbub and sloshing humanity of the real world. However, researchers have developed methods of doing this necessary work of monitoring the effectiveness of vaccines.
As you roll up your sleeve, say a silent “Thank you” to the many scientists, past and present, who worked so hard to bring about an end to this long, dark night.
¹ Please tell me that I don’t have to answer the question, “Vaccines for what?”
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