FAQs: Getting Sick After the Flu Shot

A yearly flu shot provides you with up to 60% protection against the influenza virus. The CDC recommends flu vaccines for everyone older than six months.

You might still end up getting influenza after getting a flu shot, but the vaccine does not cause it. It is impossible to be infected with the flu through the vaccine because it only contains inactive or dead type A and B influenza viruses.

Frequently asked questions about falling sick after getting the flu shot

Let us take a closer look at questions that patients might have about getting sick after the flu shot.

1. Why is the flu vaccine only partially effective?

The viruses that cause the flu mutate rapidly, making it impossible to make a vaccine that addresses all the different strains. A flu vaccine is created to address the most common strains of influenza virus spreading during that year.

The flu vaccine's effectiveness varies based on factors like the timing of the vaccine, how healthy the patient is, and how well the vaccine matches with the viruses circulating. Flu vaccines in the U.S. are usually created to protect against the four most prevalent influenza virus strains for that season.

2. Why do some people feel sick after the vaccine?

The flu vaccine sometimes leads to side effects like nausea, lightheadedness, fever, and muscle aches. These side effects are similar to the flu symptoms, so people often mistake their reaction to the vaccine with the infection. The side effects of the vaccine typically go away within two days.

The most severe side effect that can occur due to the vaccine is ab allergic reaction. For example, many vaccines are made from inactive viruses grown in eggs, which is not ideal for people who are allergic to eggs.

3. Does the flu vaccine start working immediately?

After getting the vaccine, it takes a person’s body about two weeks to build up immunity to the flu virus. A person’s immunity refers to their body’s ability to fight off illnesses. People who get the flu within two weeks of getting vaccinated were probably exposed to the virus around the same period that they were vaccinated.

4. Why should I get the vaccine if I can still get sick?

The flu shot does much more than simply prevent a person from getting the flu. It reduces the risk of being hospitalized due to the infection by up to 80%, according to the CDC. It also significantly reduces the risk of death due to the infection. People who get vaccinated typically have less severe symptoms, and they recover faster than those who are unvaccinated.