Overview of Immunization

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."

Vaccines are a way of getting your body ready to fight off certain infections. Vaccines teach your immune system 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.

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 do 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 often give them a head start in fighting the infection so they are not as sick as they would have been without the vaccine. Also, there are no vaccines for many important infections, such as HIV and other sexually transmitted infections.

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:

• One deadly disease, smallpox, has been completely eliminated

• Other serious diseases such as polio, diphtheria, and tetanus that used to be common now almost never happen in the United States

• Some common serious diseases such as measles, whooping cough (pertussis), and mumps are much less frequent.

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

• Before a vaccine can be used, it’s tested for safety

• Often, the side effects are minor, such as pain or redness where the shot was given, a rash, or a mild fever

• Very rarely, vaccines cause a more serious reaction

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.

• Babies and children usually get vaccines based on a recommended childhood vaccine schedule starting when they’re first at risk for a disease. This 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.

The following websites provide additional information on vaccines:

1. CDC: Vaccine safety FAQs for parents and caregivers

2. CDC: Vaccines for Your Children: Vaccine information provided by age

3. Vaccine Adverse Event Reporting System (VAERS): Where and how to report side effects of vaccines

4. Children's Hospital of Philadelphia: Vaccine Education Center

5. European Centre for Disease Prevention and Control (ECDC): Vaccine schedules in all countries in the EU/EEA