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

Woman coughing. France
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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 but that is just not realistic.

So Why Am I Sick? There are a number of reasons that you may get sick 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 common cold

Pneumonia (although it may protect you from pneumonia that is a complication of the flu)


Stomach flu

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.

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 understand how frustrating it can 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 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.


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


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