Making the Vaccine Decision

As a parent, you want to protect your little one from harm. Before you decide to vaccinate your baby, you may wish to know more about:

  • how vaccines work
  • how vaccines work with your baby’s immune system
  • vaccine side effects/risks
  • vaccine ingredients
  • vaccine safety

Use this page to find this information as you make the vaccine decision. If you have more questions, talk with your child’s doctor or see Infant Immunization FAQs for additional information.

How Vaccines Prevent Diseases

Smiling mother hugging baby

Ensure your baby gets her vaccines according to the CDC’s recommended schedule to give her the best protection against 14 serious diseases by age 2.

The diseases vaccines prevent can be dangerous, or even deadly. Vaccines reduce your child’s risk of infection by working with their body’s natural defenses to help them safely develop immunity to disease.

When germs, such as bacteria or viruses, invade the body, they attack and multiply. This invasion is called an infection, and the infection is what causes illness. The immune system then has to fight the infection. Once it fights off the infection, the body has a supply of cells that help recognize and fight that disease in the future. These supplies of cells are called antibodies.

Vaccines help develop immunity by imitating an infection, but this “imitation” infection does not cause illness. Instead it causes the immune system to develop the same response as it does to a real infection so the body can recognize and fight the vaccine-preventable disease in the future. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever. Such minor symptoms are normal and should be expected as the body builds immunity.

As children get older, they require additional doses of some vaccines for best protection. Older kids also need protection against additional diseases they may encounter. Learn more about vaccines for your pre-teens and teens.


Vaccines and Your Child’s Immune System

As a parent, you may get upset or concerned when you watch your baby get 3 or 4 shots during a doctor’s visit. But, all of those shots add up to protection for your baby against 14 infectious diseases. Young babies can get very ill from vaccine-preventable diseases.

The Advisory Committee on Immunization Practices (ACIPpdf icon), a group of medical and public health experts that develops recommendations on how to use vaccines to control diseases in the United States, designs the vaccination schedule. The ACIP designs the vaccination schedule to protect young children before they are likely to be exposed to potentially serious diseases and when they are most vulnerable to serious infections. This is the schedule CDC recommends.

Although children continue to get several vaccines up to their second birthday, these vaccines do not overload the immune system. Every day, your healthy baby’s immune system successfully fights off thousands of antigens – the parts of germs that cause their immune system to respond. The antigens in vaccines come from weakened or killed germs so they cannot cause serious illness. Even if your child receives several vaccines in one day, vaccines contain only a tiny amount of antigens compared to the antigens your baby encounters every day.

This is the case even if your child receives combination vaccines. Combination vaccines take two or more vaccines that could be given individually and put them into one shot. Children get the same protection as they do from individual vaccines given separately—but with fewer shots.


Vaccine Side Effects/Risks

Like any medication, vaccines can cause side effects. The most common side effects are mild. On the other hand, many vaccine-preventable disease symptoms can be serious, or even deadly. Even though many of these diseases are rare in this country, they still occur around the world. Unvaccinated U.S. citizens who travel abroad can bring these diseases to the U.S., putting unvaccinated children at risk.

The side effects from vaccines are almost always minor (such as redness and swelling where the shot was given) and go away within a few days. If your child experiences a reaction at the injection site, use a cool, wet cloth to reduce redness, soreness, and swelling.

Serious side effects after vaccination, such as severe allergic reaction, are very rare and doctors and clinic staff are trained to deal with them. Pay extra attention to your child for a few days after vaccination. If you see something that concerns you, call your child’s doctor.


Vaccine Ingredients

Vaccines contain ingredients, called antigens, which cause the body to develop immunity. Vaccines also contain very small amounts of other ingredients. All ingredients either help make the vaccine, or ensure the vaccine is safe and effective. These types of ingredients are listed below.

Vaccine Ingredients
Type of Ingredient Examples Purpose
Preservatives Thimerosal (only in multi-dose vials of flu vaccine)* To prevent contamination
Adjuvants Aluminum salts To help stimulate the body’s response to the antigens
Stabilizers Sugars, gelatin To keep the vaccine potent during transportation and storage
Residual cell culture materials Egg protein To grow enough of the virus or bacteria to make the vaccine
Residual inactivating ingredients Formaldehyde To kill viruses or inactivate toxins during the manufacturing process
Residual antibiotics Neomycin To prevent contamination by bacteria during the vaccine manufacturing process

*Today, the only childhood vaccines used routinely in the United States that contain thimerosal (mercury) are flu vaccines in multi-dose vials. These vials have very tiny amounts of thimerosal as a preservative. This is necessary because each time an individual dose is drawn from a multi-dose vial with a new needle and syringe, there is the potential to contaminate the vial with harmful microbes (toxins). Learn more about thimerosal, mercury, and vaccine safetypdf icon.

There is no evidence that the small amounts of thimerosal in flu vaccines cause any harm, except for minor reactions like redness and swelling at the injection site. Although no evidence suggests that there are safety concerns with thimerosal, vaccine manufacturers have stopped using it as a precautionary measure. Flu vaccines that do not contain thimerosal are available (in single dose vials).


Ensuring Vaccine Safety

The United States’ long-standing vaccine safety system ensures vaccines are as safe as possible. In fact, currently, the United States has the safest vaccine supply in its history.

Safety monitoring begins with the U.S. Food and Drug Administration (FDA), who ensures the safety, effectiveness, and availability of vaccines for the United States. Before the FDA approves a vaccine for use by the public, highly trained FDA scientists and doctors evaluate the results of studies on the safety and effectiveness of the vaccine. FDA also inspects the sites where vaccines are made to make sure they follow strict manufacturing guidelines.

Although scientists identify most common side effects of a vaccine in studies before the vaccine is licensed, they may not detect rare adverse events in these studies. Therefore, the U.S. vaccine safety system continuously monitors for possible side effects after the FDA licenses a vaccine. When millions of people receive a vaccine, less common side effects that studies did not identify earlier may occur.

If CDC and FDA find a link between a possible side effect and a vaccine, public health officials take appropriate action. They will weigh the benefits of the vaccine against its risks to determine if recommendations for using the vaccine should change. Learn more about the U.S. vaccine safety system.pdf icon

The Vaccine Adverse Event Reporting System (VAERS) is a national system used by scientists at FDA and the Centers for Disease Control and Prevention (CDC) to collect reports of adverse events (possible side effects) that happen after vaccination. Learn more on the VAERSexternal icon website.

Watch a video that describes this journey of a childhood vaccine from development through post-licensure monitoring.


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Page last reviewed: March 18, 2019