You are 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.
Each vaccine works to prevent only one type of infection. For example, the flu vaccine only helps prevent the flu. And you may need to get a vaccine several times for it 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 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. Also, there are no vaccines for many important infections, such as HIV and other sexually transmitted diseases.
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:
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.
Before a vaccine can be used, it’s tested for safety
Often, the side effects are minor, such as pain where the shot was given, a rash, or a mild fever
Very rarely, vaccines cause a more serious, sudden allergic reaction (called an anaphylactic reaction), such as tongue and throat swelling and difficulty breathing
Some vaccines (such as the flu vaccine) 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 when they’re first at risk for a disease, which lowers the chance they’ll get infected
Adults may need certain vaccines based on their health history, job, and location
Travelers may need certain vaccines before going to places that have diseases not normally found in their home country
Check with your doctor to find out what vaccines you need and when to get them.