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The real science behind childhood vaccines

See how vaccines work to set up your child for lifelong health.

One way to feel more confident about routine childhood vaccines is to better understand the science behind them. Learn more about what’s in them and how they work to protect your child against certain diseases.

  1. How does your body’s immune system work?
  2. How do vaccines work?
  3. Types of vaccines
  4. Vaccine ingredients
  5. Multiple doses and booster vaccines
  6. Understanding herd immunity
  7. Sources

How does your body’s immune system work? 

The immune system is like a team protecting your body from harmful germs like bacteria, viruses, and fungi. When a harmful germ enters the body, certain immune system cells quickly raise the alarm and lock onto the outside of the germ. Then, a different immune cell rushes to the scene to kill the germ.source: 1  

If this is the first time your immune system finds a particular harmful germ, the process can take time. It may take some time for the immune system to recognize the threat and build a team to destroy it. But the immune system has a great memory. So, the next time the germ enters the body, it can protect you much more quickly.source: 1   

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How do vaccines work? 

Although the immune system is amazing, it is not perfect. It takes time to build the right team to attack a specific germ. In the time it takes to do that, the germ can create its own large team that the immune system cannot fight, which leads to us getting sick. 

This is where vaccines come in. Vaccines are like teachers. They teach our immune system about harmful germs before there is a threat. They also teach our immune system to create antibodies and memory cells that fight those germs. When our immune system learns all of this from a vaccine, it is ready to jump into action. 

As soon as a harmful germ (a bacteria or virus that someone has been vaccinated against) enters our body, a vaccine can help our immune system recognize it and fight it. Vaccines help protect us from the germs and help prevent us from getting sick. Even if we still get the disease, we probably won’t get very sick thanks to vaccines.source: 1 Vaccinated people are less likely to get hospitalized or die from the disease.

Getting started with vaccine science

In your journey to better understand vaccine science, you may wonder: Where do I start? There’s so much information out there, it can be overwhelming. We’re here to help you do your vaccine homework. 

Types of vaccines

Vaccines help your immune system recognize either the whole virus or bacteria or a piece of it. Viruses and bacteria come in many different shapes, but they all have exterior “shells” and genetic material. These are what most vaccines target.source: 2

Live

These vaccines use a whole virus that is so weakened it cannot make someone sick.

MMR, Flu, Rotavirus, Varicella

Killed

These vaccines use a whole virus that is killed.

Pertussis, Rabies, Polio, Hepatitis A

Toxoid

These vaccines use toxin that is made by the germ without using the whole germ.

Diphtheria, Tetanus

Sub-unit

These vaccines use harmless pieces of the germ without using the whole germ.

Pertussis, Flu, Hepatitis A and B, Meningococcal, Pneumococcal, Typhoid, COVID-19

Virus-like particle

These vaccines use proteins that look like the germ but have no genetic material that can make someone sick.

HPV

Outer membrane vesicle (OMV)

These vaccines block the vesicle that delivers a germ’s toxins.

Group B Meningococcal

mRNA

These vaccines use messenger RNA to give the immune system a message with the blueprints to fight a certain germ.

COVID-19

Protein-polusaccharide conjugate

These vaccines work on the proteins and sugars of a germ to prevent it from making someone sick.

Hib, Pneumococcal, Meningococcal, Typhoid

Vaccine ingredients

Each ingredient in a vaccine is included for a reason. Before the U.S. Food and Drug Administration (FDA) determines that a vaccine is safe and effective, the vaccine is thoroughly studied by FDA scientists and doctors.source: 3

  • Active ingredient: Some vaccine ingredients are needed to build immunity. Vaccines have antigens, which are either a dead or weakened virus or bacteria, or a harmless piece of the virus or bacteria. Even though vaccines contain pieces of a virus or bacteria, they do not cause you to get the disease. Some vaccines also have adjuvants, which are substances that promote a strong immune response. The ingredients in a vaccine help your immune system respond to and build immunity against a specific disease.source: 4
  • Stabilizers: Other ingredients help vaccines maintain their quality and help protect the vaccine from adverse conditions.source: 5,source: 6,source: 7 It’s a long road from the manufacturing plant to the doctor’s office or pharmacy. Stabilizers help make sure the vaccine works like it should once it gets to your child.
  • Preservatives: Preservatives prevent the growth of bacteria or fungi that may get into the vaccine vial during its use. An example is when a multiple dose vial has repeated needles going into it.source: 5,source: 6,source: 7
  • Adjuvants: An adjuvant is added to some vaccines to enhance the body's immune response. One example of an adjuvant is aluminum salts. Aluminum is found naturally in the world around us. As a vaccine ingredient, it has been studied and found to be safe for more than 70 years.source: 5,source: 8,source: 9

Learn more about why vaccines have so many ingredients and how we know they are safe.

 

Multiple doses and booster vaccines

Infants’ and children’s immune systems are used to being busy. Each day, their immune systems fight off as many as 6,000 different types of antigens, or germs, that they are exposed to as they play, eat, or touch objects.source: 10 Because children's immune systems are busy fighting these antigens, they are more than able to handle multiple doses of vaccines. Each vaccine in the childhood vaccination schedule has between one and 69 antigens.source: 11 That is a lot less than the 6,000 antigens they fight off each day!

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Download : Multiple vaccines are safe

What are antigens?

An antigen is anything that causes your immune system to react. They can be bacteria, viruses, or even pollen.source 1

When an antigen causes your immune system to react from nature or a vaccine, it responds to help protect your body from reacting in the future.

Multiple vaccines at the same time are safe.

Every day your child will encounter as many as 6,000 antigens just by being a kid.source 2

And that's okay because their immune system is designed to handle it. That's why receiving multiple vaccines at one time is safe.

Even if your child got 4 vaccines at once, it would be at most 276 antigens.source 3

Much less than what they encounter every day.

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Understanding herd immunity

With many vaccines, when we vaccinate ourselves and our children, we protect others. Just like when other parents vaccinate themselves and their children, they protect our children. Many vaccines work best when everyone in a community who can be vaccinated is vaccinated. This protects the most vulnerable in our community like children who are undergoing cancer treatment or who have immune system problems.source: 12

Common questions

Vaccines go through many steps before being made available to the public. There are three important things to know about new vaccines: Vaccine science builds on decades of research; vaccine clinical trials and several other systems monitor for any long-term side effects; and misinformation about new vaccines is very common.

Learn more : How do we know new vaccines have been researched enough?

Vaccines are some of the most studied medical interventions in the world. The United States has many requirements that make sure vaccines are safe, effective, and high quality. Among these are extensive testing before approval and rigorous safety monitoring after approval.

Learn more : How do I know vaccines are safe for my child?

The Vaccine Adverse Event Reporting System, or VAERS, is a vaccine safety monitoring system and part of the national vaccine safety net. VAERS accepts and analyzes reports of possible health problems that people have after they get vaccinated. Anyone can submit a report to VAERS, and VAERS reports are available to the public. The Centers for Diseases Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) use VAERS to help identify potential safety problems with vaccines.

Learn more : What is VAERS and what does it do? 

View More Common Questions

Quiz: How well do you know the risks?

Question 1 of 5: Why do we still need to vaccinate for whooping cough?

Correct

Yes! All the answers are correct. Whooping cough can be very serious for babies.

Incorrect

Actually, all the answers are correct. Whooping cough can be very serious for babies.

Thanks to most kids getting whooping cough vaccines, cases of the disease have decreased more than 75%. And getting vaccinated while pregnant passes protection to the baby. This helps protect the baby before they’re old enough to get their own vaccine.

Question 2 of 5: If one person has measles, how contagious is it in people who are not immune?

Correct

Correct! Measles is one of the most contagious diseases in the world. If 1 person has it, 9 out of 10 people close to that person will also get measles if they aren't protected.

Incorrect

No, measles is one of the most contagious diseases in the world. If 1 person has it, 9 out of 10 people close to that person will also get measles if they aren't protected.

Measles can live in the air for up to 2 hours after an infected person leaves an area. It can be a very serious disease:

  • 1 in 5 unvaccinated people who get measles will be hospitalized
  • 1 in 20 unvaccinated children who get measles will get pneumonia
  • Unvaccinated pregnant women who get measles may have a premature or low birth weight baby

Question 3 of 5: How many childhood deaths have routine vaccines prevented?

Correct

Very good!

Incorrect

No, vaccines have saved many more children!

Vaccines are expected to prevent more than 1 million deaths among children in the United States who were born between 1994 and 2023. That’s more than the entire population of Denver, Colorado.

Question 4 of 5: What diseases have vaccines wiped from the face of the earth?

Correct

Yes! Before a vaccine was used to eradicate smallpox, 3 out of 10 people who had it died.

Incorrect

Good try, but only smallpox has been wiped from the face of the earth. Before a vaccine was used to eradicate smallpox, 3 out of 10 people who had it died.

But we are also very close to eradicating polio. Polio paralyzed almost 21,000 people in the U.S. in 1952 alone. Polio is rare in the United States, but it still exists in a few countries in Asia and Africa. Remember, we still need to vaccinate until polio is gone everywhere, because viruses, just like people, travel.

Question 5 of 5: What are the risks of serious side effects happening after vaccination?

Correct

Yes! Generally, only 1–2 vaccine doses in 1 million will result in a severe allergic reaction.

Incorrect

No, the risk is much, much smaller. Generally, only 1–2 vaccine doses in 1 million will result in a severe allergic reaction.

To put that in perspective, you have a 1 in 15,300 chance of being struck by lightning in your lifetime and a 1 in 46,744 chance of dying from a hornet, wasp, or bee sting—both much more likely than having a severe allergic reaction from a vaccine.

It’s important to understand the true risk of something to make an informed decision.

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Vaccine safety

Vaccines go through extensive testing prior to being approved and rigorous monitoring after approval to ensure they are safe for your child.source: 13,source: 14,source: 15

Learn more: Vaccine safety

Vaccine preventable diseases 

Routine childhood vaccines help prevent the spread of illnesses that once killed or harmed many infants, children, and adults.source: 16,source: 17,source: 18

Learn more: Vaccine preventable diseases 

Vaccine schedule

Sticking to the vaccine schedule helps protect your child against more than a dozen diseases.source: 19,source: 20

Learn more: Vaccine schedule

Sources

  1. HHS: Vaccines Protect You
  2. CDC: Explaining How Vaccines Work
  3. FDA: Vaccines for Children - A Guide for Parents and Caregivers
  4. HHS: Vaccine Ingredients
  5. FDA: Common Ingredients in FDA-Approved Vaccines
  6. Children's Hospital of Philadelphia: Vaccine Ingredients
  7. CDC: Vaccine Basics
  8. American Academy of Pediatrics (AAP): Vaccine Ingredients: Frequently Asked Questions
  9. CDC: Adjuvants and Vaccines
  10. AAP: All About the Recommended Immunization Schedules
  11. CDC: Multiple Vaccines at Once
  12. HHS: Vaccines Protect Your Community
  13. CDC: How Vaccines are Developed and Approved for Use
  14. FDA: Vaccine Facts: Why You and your Family Need Vaccines
  15. FDA: Vaccine Development – 101
  16. AAP: 14 Diseases You Almost Forgot About Thanks to Vaccines
  17. HHS: Five Important Reasons to Vaccinate Your Child
  18. FDA: Vaccines Protect Children From Harmful Infectious Diseases
  19. CDC: Child and Adolescent Immunization Schedule by Age
  20. CDC: Reasons to Follow CDC’s Recommended Immunization Schedule
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