Why The Flu Shot Doesn't Work (All the Time)

Why don't flu shots always work?. Jose Luis Peleaz Inc/Blend Images/Getty Images

Flu shots are quite controversial. Of all the vaccines we are asked to get, influenza is the only one that has to be given every year and has a much lower efficacy rate than most others. It's understandable that people are wary or confused about whether or not it is even worth getting.

The truth is, flu vaccines don't always work. It would be great if they did, but they don't.

Some people get the flu even after they get flu vaccines.

This occurs for one of a few reasons.

1. It could be that you were exposed to the flu before the vaccine had time to provide protection. They take 2 weeks to be effective. After vaccination, it takes that long for our bodies to develop immunities to the viruses included in the vaccine.

2. It is also possible that you got a strain of the flu that wasn't included in the vaccine. Each year, researchers do their best to determine which strains are most likely to cause illness the following year, but they don't always guess correctly. If you are exposed to a strain of influenza that is not included in one of the flu vaccines, you could still get sick.

3. Another possibility if you get the flu after you had a flu shot is that it just didn't provide complete protection for you. Unfortunately it doesn't work perfectly for everyone, so there will be some people that get the flu (even the strain included in the vaccine) after they were vaccinated.

Fortunately, most people in this category do not develop serious symptoms or complications from the flu.

4. Finally, you may not actually have the flu at all. Many people believe they have the flu when they get sick. But flu like symptoms can be caused by a lot of viruses, not just influenza.

And contrary to popular belief, the flu typically does not cause vomiting and diarrhea - or at least not as primary symptoms. Influenza is a respiratory illness. If your main symptoms are vomiting and diarrhea, you probably have a stomach virus (sometimes called the stomach flu), which is not the flu at all.


So, flu vaccines are not 100% accurate, but most things in medicine aren't. It is still the best protection we have against influenza. You can wash your hands, eat healthy, get plenty of sleep and still get the flu. You can get the flu vaccine and still get the flu as well, but you have a much better chance of avoiding it if you get vaccinated.


Key Facts About Seasonal Flu Vaccine. Seasonal Influenza (Flu) 7 Nov 13. US Centers for Disease Control and Prevention. Department of Health and Human Services. 26 Dec 13.

Vaccine Effectiveness - How Well Does the Flu Vaccine Work? Seasonal Influenza (Flu) 7 Nov 13. US Centers for Disease Control and Prevention. Department of Health and Human Services. 26 Dec 13.

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