Here's why almost no one you know got the flu this year

Keith Srakocic/AP

The 2015-2016 flu season is officially over, and it was a surprisingly mild one.

After three years of brutal influenza seasons, this year was a welcome relief.

That's mostly because the flu vaccine was a relatively excellent match for the different kinds of flu virus that were circulating the US this season.

The flu shot was about 60% effective at protecting against influenza overall, according to the US Centers for Disease Control and Prevention. 

Last year — an especially dismal year for the vaccine's compatibility — the CDC reported that the flu shot was only 19% effective overall. In other words, if you got a flu shot, your risk of going to a doctor with the flu was 19% lower. So this year marked a pretty significant improvement.

The influenza virus mutates a lot, so public-health officials have to update the flu vaccine every year in order to make it match the kinds of bugs that are currently circulating.

Picking which strains to include in each year's vaccine is a bit of a guessing game though, as officials have to try to predict (using data, of course!) which strains will be most common. This year, that calculation led to a very good match.

The flu's constant mutation is also why you're supposed to get your flu shot every year, and why last year's vaccine probably won't do you much good. This season, over 146 million Americans got their vaccine.

The more people who get the flu shot, the more protection society as a whole can enjoy. Even in years with a vaccine that has a low effectiveness, that small protection for you may not mean much — but overall, it can up the herd immunity of the general population, helping to protect the most vulnerable among us.

Hopefully, next year's vaccine will be even better than this year's.

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