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Overview of Immunization


The Manual's Editorial Staff

Reviewed/Revised Jul 2023
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What does immunization mean?

You're immune to an infection when your body's natural defenses have learned how to fight it off. You can become immune naturally after you're exposed to germs such as bacteria or viruses. Or you can become immune to a certain infection because you were given a vaccine against it. That's why getting a vaccine is sometimes called "immunization."

What are vaccines?

Vaccines are a way of getting your body ready to fight off certain infections. Vaccines teach your immune system Overview of the Immune System The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more how to fight off certain diseases. They don't fight infections after you're sick, like medicines do. Instead, vaccines help you avoid getting sick in the first place or, if you do get infected, they can help you fight the disease so you are not as sick.

Each vaccine works to prevent only one type of infection. For example, the flu vaccine only helps prevent the flu. You may need to get some vaccines several times for them to be fully effective. Because vaccines are usually given by shot (injection), several vaccines are often combined into one shot so that you get fewer shots.

Do vaccines work?

Yes, vaccines lower the risk that you will get an infection. People who don’t get vaccines are more likely to get sick or die from certain infections than people who get vaccines. However, no vaccine works 100% of the time. Some people who've gotten vaccines for certain infections can still get sick from those infections, but the vaccine can give them a head start in fighting the infection so they are not as sick. Also, there are no vaccines for many important infections, such as HIV Human Immunodeficiency Virus (HIV) Infection and AIDS The human immunodeficiency virus (HIV) is a type of virus called a retrovirus. It causes AIDS (acquired immunodeficiency syndrome), which is life-threatening. HIV is called an immunodeficiency... read more and other sexually transmitted infections Overview of Sexually Transmitted Infections STIs are infections that are passed from person to person through sexual contact, including oral sex. STIs may be caused by different types of germs, including chlamydia, gonorrhea, HIV, and... read more .

In the past, thousands of children died every year because of diseases that are now preventable by vaccines. Hundreds of thousands became seriously ill. Because of vaccines:

Are vaccines safe?

Yes, vaccines are considered to be very safe. A few people develop side effects, but the side effects are rarely serious. And the diseases vaccines prevent are more dangerous than the side effects of the vaccines.

Some vaccines (such as some flu vaccines) are made with substances from eggs. Vaccines made using eggs are more likely to cause an allergic reaction. Doctors will ask you if you're allergic to eggs before they give you one of those vaccines.

Do vaccines cause autism?

No, there is no known link between vaccines and autism.

  • Doctors from across the world have done many studies to look for a connection between vaccines and autism and didn’t find one

  • Children who get vaccines are no more likely to get autism than children who don’t get vaccines

See Centers for Disease Control and Prevention (CDC): Autism and Vaccines.

Who gets vaccines and when?

  • Babies and children usually get vaccines based on a recommended childhood vaccine schedule when they’re first at risk for a disease, which lowers the chance they’ll get infected (see CDC: Child and adolescent immunization schedule by age)

  • Adults may need certain vaccines based on their health history, job, and location (see CDC: Adult immunization schedule by age)

  • Travelers may need certain vaccines before going to places that have diseases not normally found in their home country (see CDC: Travelers’ Health)

Check with your doctor to find out what vaccines you need and when to get them.

Where can I get more information about vaccines?

The following websites provide additional information on vaccines:

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