Why You May Still Get Sick After a Flu Shot

An annual flu shot offers you the best protection against the influenza virus and is recommended for most individuals 6 months and older. It is still possible for you to get sick after a flu shot, but this isn't because of what's in the shot. The flu vaccine is made from killed, or inactivated, viruses that can't give you the flu.

This article explores the reasons why you may get sick after the flu shot. It will also explain how the vaccine may help reduce symptoms and complications if you do end up getting the flu.

Reasons You May Get Sick After a Flu Shot

Verywell / Joshua Seong

Why Can You Still Get the Flu After a Flu Shot?

There are many reasons why you may still get sick after getting the flu shot. How effective it is will depend on several factors including the timing of the shot, as well as your body's immune response.

You Haven't Built Up Immunity Yet

It takes two weeks to build up your immunity to influenza after you get the shot. Immunity is how well your body is able to fight off illnesses. If you get the flu within two weeks of getting the shot, you were probably exposed to the virus around the time you got your vaccine.

You Have A Different Illness

The flu shot does not protect against these flu-like illnesses:

Each of these illnesses can cause symptoms that may be confused with the flu.

The Correct Strain of Flu Isn't in the Vaccine

The flu shot provides protection against three or four specific strains, or types, of influenza. These strains are selected by researchers who hope to protect as many people as possible. Because the flu virus mutates, or changes, new vaccines have to be made every flu season.

Despite their best educated guesses, researchers and public health officials may get it wrong. If that year's illness-causing strains of influenza are not included in the vaccine, people who get the flu shot may still get the flu.

You Didn't Respond Fully to the Vaccine

Your immune system may not respond fully to the vaccine and you may still get the flu if:

  • You have an underlying medical condition that causes a weakened immune system, such as cancer or diabetes
  • The vaccine wasn't stored properly and as a result it's not as effective
  • The vaccine wasn't given properly and as a result it's not as effective

Even if you don't respond fully to the vaccine, you are still less likely to have serious complications from the flu. This is especially important for children and older adults who are at the highest risk of experiencing serious flu complications.

Research shows that the majority of people who are vaccinated against the flu have significantly less severe symptoms and 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 high risk for the flu and associated complications. This means that the flu is more likely to lead to very serious illnesses and even death in this age group. Despite the fact that older individuals' immune systems may not respond fully to the vaccine, it is still recommended that they get vaccinated every year.

Studies show that individuals age 65 and older who got the flu shot:

  • Reduced the risk of flu-related doctor visits by 24%
  • Reduced flu-related hospital visits by 33%
  • Reduced the risk of associated illnesses by 60%.

Summary

Despite getting the annual flu shot, it is still possible to get sick. Reasons you may get sick include:

  • You haven't built up enough immunity yet.
  • You got a different illness other than the flu.
  • The correct flu strain wasn't in the vaccine that particular year.
  • Your immune system didn't fully respond to the vaccine due to an issue with the vaccine, or an underlying health condition.
  • You're an older adult and your immune system didn't fully respond to the vaccine.

Vaccinated individuals who do get sick typically have less severe symptoms and complications. This is especially important to keep in mind for high risk groups such as individuals 65 and older, as well as children.

A Word From Verywell

It can be frustrating to get sick after getting your flu shot. Keep in mind that those who are vaccinated and do get the flu tend to experience milder symptoms, as well as reduced complications. Even if you get the flu, it's still a good idea to continue getting your annual flu shot.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Who needs a flu vaccine.

  2. Centers for Disease Control and Prevention. Misconceptions about flu vaccines.

  3. Centers for Disease Control and Prevention. Key facts about flu vaccines.

  4. Centers for Disease Control and Prevention. Selecting viruses for the seasonal influenza vaccine.

  5. Poland GA. Influenza vaccine failure: failure to protect or failure to understand? Expert Review of Vaccines. 2018;17(6):495-502. doi:10.1080/14760584.2018.1484284

  6. Centers for Disease Control and Prevention. Vaccine effectiveness: How well do flu vaccines work?

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.