Does a Breakthrough Infection Mean My Flu Shot Didnt Work?

Understanding breakthrough infections

It’s possible to contract the influenza virus after getting your flu shot. This is called a “breakthrough infection.” Many people believe a breakthrough infection is evidence that vaccination doesn’t work.

But no vaccine is 100 percent effective and, even with the best vaccines, some people are still expected to get sick after getting vaccinated.

There are a number of reasons why breakthrough infections might occur after your flu shot.

Some people have weakened immune systems due to age or underlying health conditions. This can make it difficult for their body to build a proper response to vaccination, meaning they don’t get the full protective benefit of the flu shot.

Other times, breakthrough infections may happen because a person is exposed to a strain of the virus that’s different from the ones used to develop the vaccine that year.

Each year, the World Health Organization (WHO) reviews thousands of influenza virus samples from around the world to determine which strains should be used to develop the next season’s flu vaccines. If someone is exposed to an influenza virus very different from the ones used in the vaccine, the vaccine effectiveness may be lower.

It’s also important to keep in mind that there are many seasonal viruses that cause symptoms similar to the flu, such as rhinoviruses, which cause colds, or respiratory syncytial virus (RSV). The flu shot doesn’t provide protection against other respiratory viruses.

Getting sick from the flu shot

Another common myth is that the flu shot can give you the flu. But that isn’t possible.

Flu shots aren’t made using live viruses — they contain either dead viruses (known as inactivated viruses) or a single viral protein. They don’t contain any living virus and therefore can’t give you the flu.

Nasal spray flu vaccines do contain live viruses, but these have been weakened so that they can’t replicate efficiently. In people with typically functioning immune systems, the immune response effectively controls the virus before it can cause the disease.

Then why do so many people claim to get sick from the flu shot? Often this is because of the body’s response to vaccination.

Shortly after you get the shot, it’s normal for you to experience side effects as your body mounts an immune response. Many of these mimic the symptoms of the flu, including:

  • headache
  • fever
  • chills
  • fatigue
  • muscle pain
  • nausea

The goal of vaccination is to help your body to develop an immune response as if it were encountering a real infection, but without the actual danger of getting seriously ill.

Even though you aren’t actually sick, your body acts like it’s sick. These responses are expected side effects of vaccination and are actually an indication that your body is building up immunity to the influenza virus as intended, so that if you’re exposed to the virus in future, your immune system can fight it off.

Benefits of flu vaccination

Even if breakthrough infections do occur, getting your flu shot offers many protective benefits, for you and those around you.

Reducing the severity of illness

Studies repeatedly demonstrate that the flu shot helps reduce the severity of illness in people who get the flu and reduces the likelihood of needing medical attention. In the 2019–2020 flu season, influenza vaccination prevented over 100,000 flu-related hospitalizations in the United States alone.

Preventing hospitalization is crucial during the COVID-19 pandemic, as over half of the hospitals in the United States remain under high or extreme stress.

For those who do end up hospitalized, a 2021 analysis found that across multiple different studies, getting the flu shot reduced the likelihood of being admitted to the intensive care unit (ICU) by one-quarter and the likelihood of dying from influenza by nearly one-third.

Protecting vulnerable people

The flu shot is especially important for people who are more susceptible to severe disease based on their age or health status. These include but aren’t limited to:

  • older adults (65 years or older)
  • young children
  • pregnant people
  • people with chronic lung diseases, such as asthma
  • people with heart disease, stroke, or diabetes
  • people with cancer

Research shows that getting the flu shot can help protect these people from serious complications from the flu. For example, according to a 2014 study, children who got the flu shot were 74 percent less likely to be admitted to the pediatric ICU.

Another study found that the flu shot reduced the likelihood of being hospitalized for flu-related illness by approximately 40 percent for pregnant people.

Reducing the spread of the virus

In addition to protecting yourself against the flu, getting your flu shot is an important way to protect those around you by reducing the spread of the virus.

Learn more

Along with covering your coughs, washing your hands, and avoiding people who are sick, getting your flu shot is an important tool to control the spread of the influenza virus within your community.

The takeaway

It’s important to get your flu shot to protect yourself and those around you.

If you do get sick after being vaccinated, the flu shot can help protect you against severe illness and complications, including hospitalization and death. This is especially important for people who are at risk of serious complications from the flu based on their age or underlying health status.

Getting your flu shot early can help protect you throughout the flu season. The flu shot is widely available at many pharmacies and grocery stores without an appointment.

If you have questions about whether the flu shot is right for you, connect with your doctor or local clinic to learn more.

Read more on: vaccines