Vaccines play an important role in keeping us healthy. They protect us from serious and sometimes deadly diseases. Vaccines are injections (shots), liquids, pills, or nasal sprays that you take to teach your body's immune system to recognize and defend against harmful germs. The germs could be viruses or bacteria.

Some types of vaccines contain germs that cause disease. But the germs have been killed or weakened enough that they won't make you sick. Some vaccines only contain a part of a germ. Other types of vaccines include instructions for your cells to make a protein of the germ.

These different vaccine types all spark an immune response, which helps your body fight off the germs. Your immune system will also remember the germ and attack it if that germ ever invades again. This protection against a certain disease is called immunity.

These diseases can be very serious. Because of this, getting immunity from a vaccine is safer than getting immunity by being sick with the disease. And for a few vaccines, getting vaccinated can actually give you a better immune response than getting the disease would.

As with medicines, any vaccine can cause side effects. Most of the time the side effects are minor, such as a sore arm, fatigue, or mild fever. They usually go away within a few days. These common side effects are often a sign that your body is starting to build immunity against a disease.

Serious side effects from vaccines can happen, but they are very rare. These side effects could include a severe allergic reaction. Other possible side effects are different for each vaccine. Talk with your health care provider if you're concerned about your health after getting vaccinated.

Some people worry that childhood vaccines could cause autism spectrum disorder (ASD). But many scientific studies have looked at this and have found no link between vaccines and ASD.

Every vaccine that is approved in the United States goes through extensive safety testing. It starts with testing and evaluation of the vaccine before it's approved by the Food and Drug Administration (FDA). This process can often take several years.:

• First, the vaccine is tested in labs. Based on those tests, the FDA decides whether to test the vaccine with people.
• Testing with people is done through clinical trials. In these trials, the vaccines are tested on volunteers. Clinical trials usually start with 20 to 100 volunteers, but eventually include thousands of volunteers.
• The clinical trials have three phases. The trials are looking for the answer to important questions such as
  • Is the vaccine safe?
  • What dose (amount) works best?
  • How does the immune system react to it?
  • How effective is it?
• During the process, the FDA works closely with the company who makes the vaccine to evaluate the vaccine's safety and effectiveness. If the vaccine is found to be safe and effective, it will be approved and licensed by the FDA.
• After a vaccine is licensed, experts may consider adding it to the recommended vaccine, or immunization, schedule. This schedule is from the Centers for Disease Control and Prevention (CDC). It lists which vaccines are recommended for different groups of people. They list which age groups should get which vaccines, how many doses they need, and when they should get them.

Testing and monitoring continue after the vaccine is approved:

• The company making the vaccines tests every batch of vaccines for quality and safety. The FDA reviews the results of these tests. It also inspects the factories where the vaccine is made. These checks help make sure the vaccines meet standards for quality and safety.
• The FDA, CDC, and other federal agencies continue to monitor its safety, to watch for possible side effects. They have systems to track any safety issues with the vaccines.

These high safety standards and testing help to make sure that vaccines in the United States are safe. Vaccines help protect against serious, even deadly, diseases. They not only protect you, but also help to keep these diseases from spreading to others.