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How do we know a vaccine dose is safe?

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When vaccines are studied in clinical trials, they are studied to make sure they are safe and work to prevent or reduce disease.source: 1 They are also studied to find out the right dose. The goal is to find the dose that provides a strong immune response with the fewest number of side effects.source: 2

For children, the dose is often the same for toddlers as for older children. This is because vaccines are designed to help the body create an immune response and, unlike many medicines, that is not dependent on a child’s weight.source: 3 As we age, we go through three different life stages that are associated with different vaccines and different vaccine doses:source: 1

  • Newborn: Right after birth, babies do not have a mature immune system and rely on their mothers’ antibodies to protect them.source: 4 They get these antibodies through the placenta and in their mothers’ breastmilk.source: 5 One of the reasons why most vaccines are recommended for when a baby is 2 months or older is because their immune systems don’t respond as well at first and the antibodies they get from their mother can decrease how effective a vaccine will be.source: 6,source: 7 The exception to this is the hepatitis B vaccine, which is effective in newborns and protects babies who are born to moms who have hepatitis.source: 8,source: 9
  • Childhood: This is the perfect time to get most routine vaccines because this is the time when the immune system works best. A child’s immune system is in learning mode. Vaccines at this stage of life enable the immune system to recognize and respond when children come in contact with viruses and other germs later on.source: 10
  • Adulthood: Sometimes the vaccine dose for children is less than the vaccine dose for adults. Not because the larger dose would be harmful to children—just the opposite. Adults may need a larger dose because as we age, our immune systems do not work as well as when we were younger.source: 3,source: 11

Key Evidence

  • Vaccine dose sizes are studied several times in the vaccine development process. The first time the dosage is studied is before clinical trials even start. During the “proof of concept” or pre-clinical phase, researchers work to make an immune response as effective as possible.source: 12 Part of that includes measuring different dose sizes. During Phase I and especially in Phase II clinical trials, the dose is studied in humans to see whether increasing the dose increases how well the vaccine works and if it increases adverse events.source: 2
  • Vaccines are generally studied in adults before they are studied in children. By the time a vaccine is studied in children, we already have some vaccine safety data. We know the common side effects and less common side effects.source: 2,source: 13 That is why studies in children are mostly to learn of any special considerations, like different side effects in children and what the right dose is for children.source: 14
  • Children may react differently than adults. Because children’s immune systems work differently than adults’, they may need a different vaccine dose to get the same protection.source: 11,source: 12 But it is also possible that a child may react a little differently than an adult to a vaccine. For instance, DTaP and Tdap both provide protection against diphtheria, tetanus, and pertussis (whooping cough). The difference between the two is the amount of toxoid. The lowercase “d” and “p” in Tdap indicate a lower amount of diphtheria and pertussis toxoids in the vaccines for teens and adults than DTaP, which has the full dose. The lower amount helps the immune system to continue to look for and protect against these diseases while decreasing the risk of side effects.source: 15 As children grow, their organs are in different stages of development. How quickly children absorb or break down vaccines can vary depending on the stage of this development.source: 14 All of these things are studied when developing a vaccine for children.
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Common Questions-young boy with parents

A Deeper Dive: Understanding clinical trials for children

Vaccine clinical trials typically start with adults, and trials with children are last—even if the vaccine is designed for children. This ensures there is clinical evidence about a vaccine’s safety and effectiveness before it is studied in children.source: 12,source: 16 Once research in children begins, it often starts with the teens first. After that research is conducted, research with younger children begins.source: 17

There are a lot of safeguards in place when studying vaccines in children. Clinical trials are conducted according to plans that are reviewed by the U.S. Food and Drug Administration (FDA) experts in clinical trial design. In addition to all the safety and ethical requirements for adult clinical trials, clinical trials with children have extra requirements.

Just like when planning vaccine clinical trials for both adults and children, the plans for the trials must be reviewed and approved by an institutional review board (IRB), which helps make sure the participants are protected.source: 18 If a vaccine clinical trial will include children, in order to approve the trial, the IRB must find that additional criteria are met, including the following:source: 19

  • The possibility of direct clinical benefit must justify the risk to children enrolled in a trial.
  • Parents’ or guardians’ permission for their child to participate in a clinical trial generally must be obtained after they are given detailed information, including about risks.
  • Unless the IRB grants a waiver, children must agree to participate before they are enrolled in a trial when the IRB determines that they are capable of doing so based on their age, maturity, and other factors. For instance, an infant cannot agree to participate, but most 9-year-olds are capable of providing their agreement to be in the study.

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More information

Still have questions? Talk to your child's doctor, nurse, or pharmacist.

Sources

  1. CDC: How Vaccines are Developed and Approved for Use
  2. FDA: Vaccine Development – 101
  3. CDC: 6 Things to Know about COVID-19 Vaccination for Children
  4. CDC: Reasons to Vaccinate
  5. CDC: Pregnancy and Vaccines
  6. Vaccine: Maternal antibodies and infant immune responses to vaccines
  7. CDC: Immunity Types
  8. Immunize.org: Ask the Experts: Scheduling Vaccines
  9. HHS: Hepatitis B Basic Information
  10. CDC: Reasons to Follow CDC’s Recommended Immunization Schedule
  11. Children's Hospital of Philadelphia: Vaccine Safety: Dosing Safety
  12. CDC: Developing Safe and Effective Vaccines
  13. CDC: COVID-19 Vaccine Frequently Asked Questions
  14. NIH COVID-19 Research: Ensuring Vaccine Safety for Children
  15. CDC: Diphtheria Vaccination
  16. CDC: About Vaccines for your Children
  17. American Academy of Pediatrics Selective Deposit: Enrolling Minors in COVID-19 Vaccine Trials
  18. FDA: Institutional Review Boards (IRBs) and Protection of Human Subjects in Clinical Trials
  19. HHS: Children: Information on Special Protections for Children as Research Subjects
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