This year's flu vaccine is far from perfect. But you should still get vaccinated.

1. This year's flu vaccine may not be a good match for this year's flu virus, the government says.
2. The Centers for Disease Control still recommends Americans get vaccinated.
3. Building flu vaccines is difficult because the virus mutates.

The flu vaccine doesn't match some of the circulating viruses

A strain of the flu virus infecting many Americans isn't one that this year's flu vaccine protects against, the Centers for Disease Control and Prevention said in a Wednesday bulletin.

Every year, the flu vaccine protects against a handful of different flu strains — the ones that experts expect will show up the most in the forthcoming flu season.

One of the flu strains that this year's vaccine protects against is called H3N2. That strain of flu has separate sub-strains itself — and the CDC says the virus has mutated to the point that only half the H3N2 cases now match the vaccine.

If this mutated version of H3N2 circulates widely, the CDC worries that "it could translate into reduced vaccine effectiveness against circulating H3N2 viruses."

CDC still wants you to get vaccinated

Yes, this news suggests that the flu vaccine is less effective than the CDC had initially thought. No, it doesn't mean you should give up on the vaccine altogether.

Remember: the flu vaccine still protects against about half of the H3N2 cases that have so far been detected in the United States. That means it will still likely lower your odds of getting the flu. And it's protecting against other, non-H3N2 strains of the flu too.

That being said, the CDC also recommends that doctors take additional precautions because of the mutated virus. The agency recommends that antiviral medications, like Tamiflu, be used "as a second line of defense to treat flu illness."

Flus always mutate, which makes them hard to vaccinate against

This is known as "drift": when, over time, a flu virus changes slightly as it hops from person to person. The H3N2 virus, in this case, essentially drifted away from the virus that we knew about when manufacturers created the vaccine, and when it actually got out to the public.

This is why the flu vaccine never — not this year, or other years — has a 100 percent efficacy rate. People who get the flu vaccine are at lower risk of the disease, but can also get sick because the virus is always a moving target.

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