Once more unto the breach, influenza effectiveness edition

I don’t think I’m doing a very good job of making myself clear. So I’m going to try one more time.

My concern here is that people have a rather poor understanding, in general, of what “effective” means when it comes to health care. For instance, people assume that a “good” flu shot is a lock against not getting the flu.

A meta-analysis of 17 flu-shot studies showed that when the vaccine is well-matched, 1.2% of vaccinated people were infected with influenza versus 3.9% of people not vaccinated. That makes the number needed to treat (NNT) for a well-matched flu shot to prevent one influenza infection = 37. When the match is poor or uncertain, however, the NNT is 77.*

A couple of points here. First of all, NNTs of 37 and 77 are pretty good! Especially for something like a shot. The NNT for many, many, many other things we consider “no brainers” are much higher. Also, 37 and 77, while different, aren’t so different that we’d tell you to do one, but not the other. They’re both in a class that they would be “recommended”. But, most importantly, it’s important to understand that even if you are vaccinated, you can still get the flu. It’s not like you chance went from 90% to 1%. No matter what, you still need to practice good hygiene. Wash your hands. Stay home if you’re sick. Don’t sneeze or cough on others. Common sense stuff.

My issue here is that many are treating the news of this year’s flu shot as if there’s some monstrous difference from other years, and that we should behave differently this year than other years. That’s just not the case. The difference between 60% effective and 23% effective is subtle. It’s real, but not that huge. Yes, you should get a flu shot, because the NNT is still pretty great. Herd immunity is important. But you should always, always practice good infection control as well.


*There are other studies at the link with slightly different NNTs. But this meta-analysis was the only one that compared well- and poorly-matched vaccines.

Want to know more about NNT? Here’s a video:

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