Insights on Influenza Blog
Mission: To save lives and reduce hospitalizations by protecting children, families, and communities against influenza.

Facts on Flu Vaccines

Everyone 6 months and older needs a flu vaccine every year to protect yourself and your loved ones from getting sick with the flu.

The flu vaccine is updated annually to help protect against circulating flu strains and it’s the best defense we have to fight flu.


Influenza vaccination
Ethan Jira

Ethan lost his two-year-old sister to the flu. She had not received her annual flu vaccine.

Read Gianna's Story

flu vaccine causes your body to produce antibodies which are infection-fighting cells.

How does the flu vaccine work?

The flu vaccine causes your body to produce antibodies, which help protect you from flu by fighting off germs and strengthening your immune system. These antibodies help prevent flu illness and can even help prevent serious flu-related complications like hospitalization and death. It takes approximately two weeks following vaccination for your body to build up antibodies against the flu.

How effective is the flu vaccine?

Flu vaccines can reduce the risk of flu illness by up to 60%.1 Even flu vaccines that aren’t well-matched to circulating strains can make flu symptoms less severe if you do get sick. But flu vaccination benefits don’t end there. People who are vaccinated are less likely to be hospitalized or die from flu-related complications.

An annual flu vaccine is the first and most important step to prevent flu.


Illness from Flu


Flu Symptoms


Flu-Related Complications

Noel Smith and Influenza Vaccination

Noel’s parents had tried to get her a flu shot several times, but a string of events prevented that from happening.

Read Noel's Story

How do we know that flu vaccines are safe?

Flu vaccines have been used for more than 50 years with very good safety records. The CDC and the FDA continuously monitor the safety of all vaccines, including the flu vaccine. New vaccines go through years of research and clinical trials before they can be deemed safe and effective by the FDA and made available to the general public.

You can't get flu from a flu vaccine

Can you get the flu from the flu vaccine?

Absolutely not. To cause infection, flu viruses need to reproduce, i.e., make copies of themselves. Flu vaccines, regardless of how they are made, do not contain viruses that can reproduce and make you sick with the flu.

If you should happen to feel ill following flu vaccination, there are a few possible explanations.

  • You may be suffering with another infection that is not flu.
  • You may be experiencing side effects of the vaccine, which should not be confused with actually being infected with the flu virus. A slight fever, aches or fatigue can be a sign that your immune system is being activated to build immunity in response to the vaccine. This is not flu. When someone is sick with flu they suffer severe and long-lasting symptoms.
  • Since it can take up to two weeks following a flu vaccine for your body to build the proper immune response to help prevent flu, you could have been exposed to the flu virus before you were protected by the vaccine.
Vaccinate to protect your family

Parker's mom gets him a flu vaccine every year because flu took the life of the sister he will never know. 

Read Cayden's Story

What are possible flu vaccine side effects? 

Most people who receive the flu vaccine have no reaction. Some people may experience redness and slight swelling at the site of injection, fever, headache, and/or muscle aches. These side effects are actually evidence that your body is responding to the vaccine to fight off a future flu infection. The risk of a severe allergic reaction in those who are vaccinated is less than one in one million.2

Risk of a severe allergic reaction in those who are vaccinated is less than 1 in 4 million


There are different types of vaccines such as:

Influenza Vaccination

Inactivated vaccines contain a dead/killed virus that are administered as a “shot” with a needle.

Influenza Vaccination

Image source: CDC, Douglas Jordan, M.A., image provider.

Nasal spray vaccines contain a live, weakened flu virus that is sprayed into the nose instead of being administered as an injection.

Global Flu Strands

Who decides which strains go into the flu vaccine? 

The Centers for Disease Control and Prevention (CDC), the U.S. Food and Drug Administration (FDA), and the World Health Organization (WHO) conduct year-round, global surveillance on circulating flu strains in order to select the flu strains that are included in the flu vaccine each year.

What’s in the flu vaccine?

Depending on the manufacturer, there can be different ingredients in a flu vaccine. All the ingredients in a flu vaccine are necessary to elicit an immune response and are extensively tested for safety. Here’s a list of some general ingredients that may be found in a flu vaccine:


Proteins from eggs or animal cells used to grow the flu virus may be present in the vaccine.


Stabilizers such as sugars or gelatin may be used to ensure that the vaccine doesn’t lose its effectiveness during transport or storage.


Antibiotics may be used to prevent contamination by bacteria during the vaccine manufacturing process.


Preservatives may be used in multi-dose flu vaccine vials to prevent contamination by dangerous germs like bacteria and fungi. Thimerosal, which is used as a preservative only in multi-dose flu vaccine vials, contains ethylmercury (not to be confused with the toxic form of mercury called methylmercury, which can be found in certain kinds of fish). Ethylmercury is readily eliminated by the body and does not build up to harmful levels. To learn more about thimerosal, visit the CDC’s website here.


Adjuvants may be used to stimulate the body’s response to a vaccine. Not all flu vaccines contain adjuvants. Aluminum salts have been used as vaccine adjuvants for decades and been found to be safe. For more information on adjuvants in vaccines, visit the CDC’s website here.


Inactivating agents are used to inactivate the flu virus that has been harvested from eggs or animal cells. Formaldehyde is an inactivating agent commonly used in flu vaccines. Residual amounts of formaldehyde may be present in a flu vaccine, but the amount is comparable to, or even less than, the amount naturally found in our bodies for human metabolism and building proteins. For more information on formaldehyde in vaccines, visit the Children’s Hospital of Philadelphia’s website here.

Can I get a flu vaccine if I have an egg allergy?

Because certain flu vaccines contain a small amount of egg protein, people with egg allergies were once advised to avoid it. However, the guidelines have changed and here is what you need to know:

  • People with a history of egg allergy who have experienced only hives after exposure to egg should receive the flu vaccine.
  • People who report having had reactions to egg involving symptoms other than hives; or who required epinephrine or another emergency medical intervention, may receive a flu vaccination administered in an inpatient or outpatient medical setting, including, but not limited to hospitals, clinics, health departments, and physician offices. Vaccine administration should be supervised by a health care provider who is able to recognize and manage severe allergic conditions.
  • A previous severe allergic reaction to flu vaccine is a contraindication to future receipt of the vaccine.

Please visit the CDC’s website here for a full explanation of flu vaccination in patients with egg allergies.

Blake Crane

Flu took the life of 16-year-old Blake Crane, causing him to die of cardiac arrest.

Read Blake's Story

When is the best time to get a flu vaccine?

When to get the influenza vaccination

Medical experts recommend that people get their flu vaccine by the end of October, and ideally before flu starts circulating in their community.

The flu virus tends to spread in the U.S. from October to May, with peak activity generally occurring in January and February. However, vaccinations can be given at any time during the flu season and getting a vaccination later in the season (December through March) can still help protect you from influenza. But remember, it takes your body up to two weeks to build up immunity following flu vaccination.

Even if you've had flu during the season, it is still recommended that you get a flu vaccine since getting one strain of flu does not prevent you from getting any of the other circulating strains.3

Why is it suggested to get a flu vaccine every year? 

Because flu viruses are constantly changing, flu vaccines may be updated from one season to the next to protect against the most recent and most common circulating flu viruses. In addition, a person’s immune protection from the vaccine declines over time; therefore, annual vaccination is needed for optimal protection.

Chance and Flu Vaccination

Chance's family did not know that the vaccine changes annually and as such people need to get vaccinated every year.

Read Chance's Story

Where can I get a flu vaccine?

Flu vaccines are readily available in a wide variety of locations, including doctors’ offices, schools, workplaces, supermarkets, pharmacies, health departments and community centers. 

Each state has laws that govern the age at which pharmacists can administer flu vaccination, so if you are seeking a flu vaccination for a child, some locations may not be able to assist you.

To find a flu clinic near you, use our flu clinic locator:


Looking for a flu vaccine? Enter your zip code here:

Visit these other pages to learn more:

Page last reviewed: September 2021. 

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