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How Effective Are Flu Shots?

It varies greatly year to year.

This year's flu vaccine is not as effective as the Centers for Disease Control and Prevention had hoped.

CDC and the United Nations' World Health Organization make their annual predictions for the North American flu season in February to give pharmaceutical companies time to ramp up production of that formula. In March, a variant of the virus CDC and WHO had targeted also began to appear. But by then, it was too late to include the variant in vaccines. Now, "roughly half of the H3N2 viruses analyzed are drift variants," CDC said in a statement last week. The current vaccinations still offer some protection, but they will be less effective against this altered strain. Compounding the problem: H3N2 strains tend to be particularly harsh among older populations. A bad flu season may be underway.

So should you still get a flu shot? Yes.

The two organizations do their best to predict which strain of the flu will come to infect the most people during the flu season. But it's just that—a guess.

A 2011 meta-analysis in the journal Lancet: Infection found significant variation in vaccine effectiveness season-to-season. Analyzing placebo-controlled studies of the two most common types of flu vaccines over the past 20 years, the authors concluded that "evidence for consistent high-level protection is elusive for the present generation of vaccines." Overall, the efficacy for the most common type of flu vaccine used in adults is around 59 percent. In a good year, reports the Los Angeles Times, a flu vaccine can be 70 percent effective. Last year's crop was around 50 percent effective.

According to Science Based Medicinea blog that plays watchdog over pseudoscience on the Internet (they have a lot of work to do)—this year the flu vaccine will protect against 57 percent of the strains circulating in the population. "Unfortunately, until there is a universal vaccine that targets the parts of the virus that don't mutate so rapidly, the flu vaccine will always be suboptimal," Science Based Medicine writes.

So should you still get a flu shot? Yes. Even though the formula is not a perfect match, it will likely help lessen the severity of the illness. And other than the needle prick, it won't hurt.