The flu vaccine protects against three or four influenza virus strains that research indicates will be circulating during the upcoming flu season.
The flu vaccine causes your body to produce antibodies which are infection-fighting cells. These germ-fighting antibodies strengthen your body's immune system and help prevent infection and serious illness when you are exposed to flu germs. It takes approximately two weeks after vaccination for your body to build up protection against the flu. Because flu vaccines use a killed or weakened version of the flu virus, they can't make you sick.
When well-matched with circulating flu strains, the flu vaccine has been shown to reduce the risk of flu illness by up to 60 percent.1 Even if the flu vaccine is not well-matched to the circulating flu strains for a given year, it still can offer protection by making flu symptoms less severe if you do get flu. It’s also a critical way to reduce the risk of flu-related complications that can lead to hospitalization or death.
An annual flu vaccine is the first and most important step to prevent flu.
Illness from Flu
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.
Absolutely not. To cause infection, flu viruses need to replicate. Flu vaccines contain inactivated (dead) or attenuated (weakened) flu viruses, or no flu virus at all, so it's biologically impossible for a flu vaccine to make you sick. Weakened flu viruses are cold-adapted and meant to replicate at cooler temperatures within the nose, but cannot cause infection in the lungs or other areas of the body where warmer temperatures exist.
Most people who receive the flu vaccine have no reaction. Up to 25 percent may have some redness and slight swelling at the site of injection. Common side effects can include soreness, redness and/or swelling at the injection site, fever, headache, and/or muscle aches. These common side effects are actually evidence that your body is having an immune response, which is what it’s supposed to do! The risk of a severe allergic reaction in those who are vaccinated is less than one in one million.2
There are different types of vaccines such as:
Inactivated vaccines contain a dead/killed virus that are administered as a “shot” with a needle.
Nasal spray vaccines contain a live, weakened flu virus that is sprayed into the nose instead of being administered as an injection.
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.
Depending on the manufacturer, there can be different ingredients in a flu vaccine. All the ingredients in a flu vaccine are necessary to illicit 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.
- 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.
When is the best time to get a flu vaccine?
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
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.
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:
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Page last reviewed: July 2020.