Government officials typically say the flu shot is about 50-60 percent effective in preventing influenza. We’ve used that number ourselves, as recently as this month in the usual annual promo for flu vaccines. But health care researchers who count actual patients say it’s far, far less effective, more in the range of 1-3 percent.
What’s the difference between the competing numbers? And why is the difference so big? The source of the 50-60% effective number is actually shrouded in mystery; the Centers for Disease Control and Prevention, which uses the number a lot, has no specific source. The best guess is it’s a “relative risk reduction,” which means that if your risk was, say, one in 100 before vaccination, and then was one in 200 after, that’s a drop of 50% if you divide those two numbers (100/200). But in terms of what doctors call “absolute” risk, it’s a drop of only half a percent: from 1% (1 in 100) to 0.5% (1 in 200).
The point is you need to know actual numbers of human beings who are saved from a miserable bout of flu by the vaccine, not just some relative percentage comparison which sounds more impressive than it really may be.