This flu season, national health authorities made a relatively poor guess of which flu strain would be going around. As a result, the media has endlessly repeated a study finding that this year's flu vaccine is only "23 percent effective." Offhand, you might guess that this means only 23 percent of vaccinated people are protected against the flu — or maybe you could somehow get one-fourth of a case of the flu?
Not quite. It turns out this definition is quite complicated, and isn't even a measurement of the general population, but instead of people with acute respiratory infections. Aaron Carroll carefully explains the details below, but the bottom line is that even when the flu shot isn't a great match, it's still worth getting. —Ryan Cooper
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