The CDC announced last week that this year’s flu vaccine was only 23% effective, but most of the coverage has not been great about explaining what that even means.
There are two ways in which flu vaccines can fail. The first is that they can just fail to give you the immunity they’re going for. A vaccine works by triggering an immune response, so that your immune system will recognize the wild germs and know all their wily secrets and be able to pummel them into submission before you ever get sick. However, sometimes this just doesn’t happen: your immune system ignores the weakened virus and fails to note down its characteristics (or does a half-assed job) so later, if you run into the virus, you’ll still get sick.
The second failure point — and this was the big problem this year — is picking the strains to vaccinate against. They have to start vaccine production in the spring to have it ready in the fall, and this year, one particular flu strain went crazy with the mutations and so if you got the vaccine, even if it worked perfectly, you may be immune to a bunch of strains you will never actually encounter while remaining mostly non-immune to a bunch of strains that will be sneezed all over you the next time you step outside your house.
So. What the “23% effective” thing actually means, according to the CDC: if you got the flu vaccine this year, despite all the different ways it can fail to work for you, you are (overall) 23% less likely than the people who didn’t get the vaccine to wind up in a doctor’s office with the flu.
Except, if you’re reading this, you’re probably over 18, and the CDC thinks the vaccine may work better for children than for adults. They even provide a bunch of H3N2-specific numbers before saying they’re “not statistically significant” so I’ll just skip over them rather than getting into the depressing data about how very, very ineffective the vaccine was this year for adults like me. I mean, if they’re not statistically significant then I don’t need to worry, right? H3N2 is the strain that started industriously mutating last April, the one you’re most likely to be exposed to and the least likely to be immune to.
The CDC still wants everyone to get a shot, if they haven’t already, because while from an individual perspective, 23% may be “why even bother” territory, from a public health perspective it’s still “way, way better than 0%, guys,” although I am not sure whether they are taking into their calculations the fact that in order to get the flu shot, you might wind up in a doctor’s waiting room full of people with the flu. (Though you could also go to your local pharmacy, since most of them have flu shots on site. There are almost certainly people in your Target who have the flu right now, but you are probably not letting that deter you from going to Target.)
The other big question that some of the articles are skipping over: so how effective is the flu shot normally? It varies by year but is normally around 50% effective. In other words, most years, getting the vaccine makes you about half as likely to wind up in a doctor’s office with the flu as the people who didn’t get the vaccine. Something fun I noticed about that effectiveness chart: they left out the year that the H1N1 epidemic hit! Presumably because the seasonal flu vaccine that year was 100% ineffective against H1N1, although a lot of adults, including probably me, had some lingering protection from when the “Russian flu” went around in 1977-78. At least, that was my theory on why my kids got it, and I didn’t.)
What should you do? Wash your hands a lot. Hope that the people with the flu stay the hell home or at least cover their coughs and sneezes (with your elbow, people! That’s what we teach all the little kids to do these days because you do not open doorknobs with your elbow.) And get the flu shot, because a 23% reduction in the odds of feeling like boiled roadkill for five to fourteen days really is better than nothing. (But, you know, maybe get it somewhere that you were planning to go anyway.)
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