Why You May Still Get Sick After a Flu Shot

Many of us have heard stories of people who still got sick even after getting a flu shot, or maybe this has even happened to you. However, this doesn't mean your illness is necessarily the flu itself. While the flu shot is a great and sometimes life-saving defense against the most common strains of influenza virus, it will not protect you from all respiratory illnesses. There are a number of reasons that may explain why you still got sick after you get a flu shot. Read on to learn more.

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; therefore, 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 that 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 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 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. Because people in this age group are at high risk for severe complications from the flu, it is also very important for those who care for them to be immunized.

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.

Regardless of your past experiences, it is always a good idea to get vaccinated to decrease your chances of getting the flu or giving it to someone who is at high risk, unless your healthcare provider has told you that you should not.

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Article Sources

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  5. Centers for Disease Control and Prevention. Influenza (flu). Adults 65 & over.