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You can still get the flu, even if you got a flu vaccine. The flu shot lowers your chances of getting influenza, and if you do get the flu, it's likely to be less severe.
There are over a hundred strains of influenza. The shot usually protects against the top three or four strains, so there are a number of strains not covered. However, some of these strains are close enough that the flu vaccine will still offer some protection. Sometimes, the predictions are wrong, and the wrong strains are in the flu shot for that particular year. In addition, not everyone who gets the shot will be immunized. These people are called non-responders. This changes from year to year, but around 90 percent of people will respond to the shot, so there are some people who even though they got the shot are not protected.
The ability of flu vaccine to protect a person depends on two things: 1) the age and health status of the person getting vaccinated, and 2) the similarity or match between the virus strains in the vaccine and those circulating in the community. If the viruses in the vaccine and the influenza viruses circulating in the community are closely matched, vaccine effectiveness is higher. If they are not closely matched, vaccine effectiveness can be reduced.
However, it’s important to remember that even when the viruses are not closely matched, the vaccine can still protect many people and prevent flu-related complications. Such protection is possible because antibodies made in response to the vaccine can provide some protection (called cross-protection) against different, but related strains of influenza viruses.
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