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Why It's So Hard To Get A Flu Vaccine Right

There's something to those headlines that call this flu season worse than usual. Annual vaccination efforts are a sort of arms race: Some years, vaccine makers can't keep up with the flu.

Every year, researchers compose a cocktail flu vaccine that inoculates people against three or four strains. Health officials have to make an informed prediction about which strains will show up in the next season.

But flu is notorious for mutating. When it rearranges the proteins on its surface, vaccines against it are less effective. And this happens quickly. Sometimes, by the time researchers grow the year's vaccines in chicken eggs, those vaccines can be out of date.

Certain strains, like the H3N2 virus that's making the rounds this season, also mutate more quickly than others and can lead to more severe illness.

SEE MORE: The Flu Nearly Reached Epidemic Levels This Season

So if the vaccine predictions are wrong, the shot will be less effective. But officials urge everyone to get vaccinated every year anyway. The benefits still outweigh any downsides — and in the years where it's matched well, a vaccine can be as much as 60 percent effective.

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