Does the Flu Shot Cause the Flu? Why Some People Feel Sick After Getting Vaccinated

When you get your flu shot this year, you may notice you don’t feel so good the morning after. A slight fever? Aching muscles? Isn’t the flu vaccine supposed to protect you from the flu, not give it to you?

There are, of course, several things wrong with the idea that vaccines cause the illnesses they are manufactured to prevent. But while vaccines can have side effects, it’s important to understand how they work — and why some people get sick even after a shot.

What is a flu shot made of?

Flu vaccines are either made from inactive or weakened live viruses, depending on the method of manufacturing and delivery. Flu shots expose you to the inactive virus, while the nasal spray method introduces a live, weakened virus into your system.

The vaccines contain a variety of other ingredients, ranging from egg proteins to antibiotics to formaldehyde (in trace amounts that won’t endanger your health).

Vaccines are specifically designed to protect you from the flu without exposing you to a virus that will make you sick. Vaccines “train” your immune system to resist a particular disease so it’s ready to fight it if you come into contact with it again.

Does a flu shot give you the flu?

Flu shot

Flu shot | -/AFP/Getty Images

No. Because flu vaccines don’t contain an active virus, it’s impossible to contract the illness from the vaccine itself.

It’s possible to experience symptoms such as a headache, muscle aches, and a low-grade fever after getting vaccinated. But these are not flu symptoms.

However, it’s also possible to still get sick even if you’ve been vaccinated. But not for the reasons you might think.

Can you still get the flu after a flu shot?

Yes. Some people do get sick with a flu virus despite getting their annual flu shot. But this isn’t because the vaccinations don’t work for most people.

According to the CDC, people who become sick after they get vaccinated were either exposed to the virus before the vaccine took effect or were exposed to a virus the vaccine wasn’t designed to protect against. Some people come down with respiratory viruses that exhibit flu-like symptoms but are not actually the flu.

Sometimes, some individuals simply aren’t protected from the virus despite getting vaccinated. But officials warn that it’s still better to get a flu shot than risk getting the flu without one. You’re much more likely to experience a bad reaction to the flu than you are to react negatively to the vaccine.

How long does a flu shot last?

Vaccine in vial

Vaccine in vial | iStock.com/Mckyartstudio

Experts advise everyone older than 6 months should get vaccinated annually.

The reason you have to get a flu shot every year is that it protects you less and less as time goes on. Your immune response declines, leaving you vulnerable again as if you never received the vaccine. However, flu viruses also change, so the vaccines themselves are slightly different from year to year.

This is the same reason doctors recommend you get vaccinated in early fall instead of waiting until peak flu season to get protected.

If you’re looking for a location near you that offers flu vaccines, check your local pharmacy. You can also get one from your primary care doctor.