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

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Many of us have heard stories of people who still got sick even after getting a flu shot. Perhaps you yourself got the flu shot only to come down with something anyway. 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 illness. There are a number of reasons that you may still get sick even after you get a flu shot.

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:

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.

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 o​f 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 percent to 70 percent 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 percent to 60 percent effective at preventing hospitalizations from pneumonia and the flu, and up to 80 percent 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 percent of flu infections in young children, with the numbers being even higher for older children.

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 health care provider has told you that you should not.

Sources:

"Vaccine Effectiveness - How Well Does the Flu Vaccine Work?" Seasonal Flu. Centers for Disease Control and Prevention. 2016.

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