Why You Can Still Get the Flu—Even If You Got a Flu Shot

The seasonal flu is typically caused by two types of the influenza virus and their related classifications, including:

• Influenza A - Type A influenza is the most common cause of flu and is spread from person to person. The A subtypes that commonly affect people are A(H1N1) and A(H3N2) according to the Centers for Disease Control and Prevention (CDC).
• Influenza B - Type B influenza is less common but still spreads among humans. It’s dangerous but less severe than Type A. Type B influenza is classified by lineages, including B/Yamagata and B/Victoria.

The flu shot protects against both A influenza viruses and at least one B influenza—sometimes both. Since influenza viruses change rapidly, the flu shot is developed to mimic the virus strains most prominent in other parts of the world during the six months before the flu season in the U.S. 

The flu shot is generally an effective way to protect yourself from the flu because it helps your body develop necessary antibodies to fight back against the virus. When the flu shot is well matched to the influenza strains that are circulating, you are less likely to get the flu. In fact, the CDC says the flu shot reduces your chances of catching the flu by 40% to 60%.

Related Article: Who needs a flu shot, and how does it work?

However, it’s not a perfect science, and some years, the flu shot doesn’t match as well as we would hope. For example, in 2017, the flu strain mutated right in the middle of the season. As a result, millions of people got sick with the flu, and 80,000 Americans died, according to the CDC.

It takes two weeks to kick in.

Another reason you can still get the flu even if you got a flu shot is that it takes time for your body to build up the antibodies it needs to fight off the flu. In most cases, this can take nearly two weeks. If you come into contact with someone with the flu virus before your body has developed enough antibodies, your immune system may not protect you from the flu.

Related Article: Learn about the differences between a cold and the flu.

Other factors may affect your risk of getting the flu.

Age can greatly affect how effective the vaccine is in reducing your risk of getting the flu. Infants and young children are at an increased risk of catching the flu because their immune systems haven’t had enough time to build up the antibodies they need to fight off a new infection.

Similarly, older adults may have a declining immune system as a result of other health conditions, such as diabetes, kidney or liver disease, or cancer. The CDC offers a higher dose vaccine for some adults over the age of 65 to help strengthen their immune system’s response.

While the flu shot can’t eliminate your risk of coming down with the flu, it does offer many benefits, including greatly reducing your chances of getting sick.

And if you do get the flu after getting vaccinated, you’re more likely to get a milder case with less severe symptoms. The vaccine also lowers the likelihood of serious flu-related complications, like pneumonia or being hospitalized.

Take additional precautions to protect yourself against the flu.

An annual flu shot is the best way to protect yourself against the flu. But since you can still get the flu even if you get the flu shot, there are additional things you should do to minimize your chances of getting sick.

• Wash your hands often.
• Avoid touching your eyes, nose, or mouth.
• Practice healthy habits, like getting plenty of sleep, exercise, water, and nutrients.