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How Effective Is the Flu Vaccine?

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But the sooner you get it, the better. Why? Simple: the further you get into the flu season, the higher your risk of getting flu. Here’s something else to keep in mind: it can take two weeks for the flu vaccine to take effect. So if you’re exposed to the flu within that two week period, you might still get sick.

  • How Well the Vaccine Is Matched With the Dominant Flu Strains

Unlike other vaccines, the flu vaccine has to be redesigned each year. That’s because the dominant strains of the flu change every flu season. Once a flu season is over, the old vaccine is worthless.

The tricky part is that researchers have to create the vaccine long before the flu season starts. So they can’t know, for sure, what strains will be dominant when the flu season actually arrives. They make predictions. While those predictions are generally accurate, they aren’t foolproof. The effectiveness of the flu vaccine in a given year depends on how accurate these projections are.

Unfortunately, getting the flu vaccine isn’t a guarantee that you won’t get the flu. But that doesn’t mean you should skip it. 

This is especially true of people who are at high risk for flu complications. Even though the flu vaccine may not work quite as well in young children, older adults, and the ill, these very same people are the most likely to have severe and even life-threatening complications from the flu. It’s crucial that they get vaccinated. While it may not be perfect, the flu vaccine is the best defense we have.

One more thing to keep in mind: the flu vaccine does not protect against cold viruses. Some people believe that the flu shot doesn’t work because they get sick despite being vaccinated. But in most of these cases, experts say, the flu vaccine did work -- it’s just that these people came down with an unrelated cold virus.

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WebMD Medical Reference

Reviewed by Kimball Johnson, MD on July 25, 2012

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