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Why Did I Get Sick After I Had a Flu Shot?

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Updated February 18, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Question: Why Did I Get Sick After I Had a Flu Shot?

I frequently hear people complain they still got sick even after getting a flu shot. As a nurse, this is a frustrating thing to hear. Some people seem to expect flu shots to protect them from every type of respiratory illness out there. That is just not realistic.

Answer:

There are a number of reasons that you may get sick after you get a flu shot.

  1. The vaccine did not have time to provide full immunity.

    It takes two weeks to develop immunity to influenza after you get the vaccine. If you get the flu within two weeks of getting the shot, you were probably exposed to the virus right before or right after you were vaccinated.

    It is easy to see why someone would believe the flu vaccine gave them the flu right after receiving the vaccine. However, the vaccine is made from killed (shot) or inactivated (nasal spray) virus and can't give you the flu.

  2. You have another "flu-like illness."

    The flu shot does not protect against:

    The flu shot does provide protection against the specific strain of the flu that researchers believe will be causing illnesses that season for most people. Every year, the flu virus mutates and changes; therefore, new vaccines have to be made and administered each season.

  3. You didn't respond fully to the vaccine.

    It is still possible to get the flu after having a flu shot, either because you were one of the few people who was not fully protected or because the strain of influenza that made you sick was not included in the vaccine. However, you are less likely to have serious complications from the flu if you have had the shot. This is even more true for older adults and children, two groups that are at highest risk from serious flu complications.

    Flu shots work in slightly different ways for these two groups, but they are still very important.

    Older Adults and Flu Shots

    Anyone over the age of 65 is considered to be in a high-risk category and should have a flu vaccine every year. The vaccine is not quite as effective at preventing the flu in this age group. However, among older adults who do not have chronic illnesses and who do not live in nursing homes, the shot is 30% to 70% effective at preventing hospitalizations from pneumonia and the flu.

    Among older adults who do live in nursing homes or have chronic illnesses, the vaccine is 50% to 60% effective at preventing hospitalizations from pneumonia and the flu, and up to 80% effective at preventing death from the flu.

    Because people over age 65 are at high risk for severe complications from the flu, it is also very important for those who care for them to be immunized.

    Children and Flu Shots

    The other group of people at highest risk for serious complications from the flu is children, especially those under 5 years old. Children under 6 months old are at highest risk for complications from the flu, but they are too young to receive the vaccine. For this reason, it is very important for parents and caregivers of infants to get the flu vaccine. The vaccine can prevent up to 66% of flu infections in young children, with the numbers being even higher for older children.

 

Bottom Line: Get Vaccinated

I can certainly understand how frustrating it must be to develop a significant respiratory illness the same year you get the flu shot. But that does not necessarily mean it didn't work. Even if you actually got the flu, that doesn't mean the shot won't work for you in the future. Regardless of your past experiences, it is always a good idea to get the flu shot to decrease your chances of getting the flu or giving it to someone who is at high risk, unless your doctor has told you that you should not have it.

Because I have heard this complaint so many times, I am interested in hearing your stories. Please share your experiences with the flu shot and illnesses you may have experienced later that season in the Cold and Flu Forum. Have these experiences kept you from getting a flu shot since then? Do you still have more questions about whether or not the shot is really safe and effective? Share your thoughts and add to the discussion.

Sources:

"Vaccine Effectiveness - How Well Does the Flu Vaccine Work?" Seasonal Flu 13 Oct 11. Centers for Disease Control and Prevention. 22 Nov 11.

 

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