Why You May Still Get Sick After a Flu Shot

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The flu shot offers you the best protection against influenza, which is why the Centers for Disease Control and Prevention (CDC) recommends the annual seasonal flu vaccine for everyone over age 6 months. That said, it's still possible for you to get sick after a flu shot—but not because of what's in the shot itself.

A flu vaccination can't shield you from all respiratory illnesses. Furthermore, its effectiveness depends on several factors, including timing, your body's immune response, and others.

Reasons You May Get Sick After a Flu Shot
 Illustration by Joshua Seong. © Verywell, 2018.

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.

You Have Another Flu-Like Illness

The flu shot does not protect against:

It is still possible—and quite likely—that you will get sick at some point during flu season with some other illness that you might mistake for the flu. Just because you had a flu shot, that does not mean you will not get sick at all. You might have a similar illness that is caused by a virus other than influenza.

The Correct Strain of Flu Isn't Included in the Vaccine

The flu shot provides protection against the specific strain of the flu that researchers believe will be causing illnesses that season for most people. Unfortunately, this doesn't provide coverage for all possible influenza strains, and the flu virus mutates and changes every year (why new vaccines have to be made and administered each season).

Sometimes, despite their best efforts and educated guesses, researchers and public health officials get it wrong. During flu seasons when the primary strain of influenza that causes illness is not included in the vaccine, many people who get the flu shot will still get the flu.

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.

Even so, 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—the two groups that are at highest risk for serious flu complications. Flu shots work in slightly different ways for these two groups, but they are still very important.

Ultimately, research has shown that a majority of people who are vaccinated against the flu have significantly less severe symptoms and fewer complications when they get sick than those who are unvaccinated.

You're Over the Age of 65

Anyone over the age of 65 is considered to be in a high risk category and should have a flu vaccine every year (the same goes for their caretakers). This is despite the fact that the vaccine is not quite as effective at preventing the flu in this age group.

Among older adults who do not have chronic illnesses and who do not live in nursing homes, the shot is 40% to 70% effective at preventing flu-related doctor visits.

Older adults who do live in nursing homes or have chronic illnesses have a 50% to 60% higher chance of being hospitalized from pneumonia and the flu. Additionally, 70% to 90% of seasonal flu-related deaths have occurred in people over age 65.

A Word From Verywell

It can be frustrating to develop a significant respiratory illness the same year you were proactive and got the flu shot. Remember, however, that getting sick does not necessarily mean the vaccine didn't do its job. And even if you actually do get the flu, that doesn't mean the shot won't work for you in the future.

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Article Sources
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  1. Centers for Disease Control and Prevention. Who needs a flu vaccine and when. Updated October 11, 2019.

  2. Centers for Disease Control and Prevention. Key facts about flu vaccines. Updated December 2, 2019.

  3. Centers for Disease Control and Prevention. Misconceptions about flu vaccines. Updated September 25, 2019.

  4. Centers for Disease Control and Prevention. Selecting viruses for the seasonal influenza vaccine. Updated September 4, 2018.

  5. Centers for Disease Control and Prevention. Influenza (flu). How well flu vaccines work. Updated January 3, 2020.

  6. Centers for Disease Control and Prevention. People 65 years and older and influenza. Updated November 21, 2019.