Guide
Vaccine glossary for parents
Plain-language meanings for the terms used on this site and in a conversation with your child's healthcare practitioner. Each term links from the pages where it appears.
Key takeaways
- A minimum age and a minimum interval are the earliest a dose counts; waiting longer is always allowed and never restarts a series.
- The CDC counts a dose given up to 4 days before a minimum age or interval as valid.
- Shared clinical decision-making means a vaccine is available after a family and clinician talk it through, rather than recommended by default.
- Exemption and provisional-enrollment rules are set by each state.
- AAP (American Academy of Pediatrics)
- A professional organization of pediatricians. It publishes its own recommended childhood immunization schedule each year. In 2026 that schedule differs from the CDC's on a few vaccines. CDC vs. AAP in 2026 →
- ACIP (Advisory Committee on Immunization Practices)
- A federal advisory committee that develops recommendations on the use of vaccines in the civilian population of the United States.
- Adjuvant
- An ingredient added to some vaccines to help the body build a stronger immune response to the vaccine's active part. Not every vaccine has one.
- Aluminum adjuvant
- Aluminum salts used as an adjuvant in some childhood vaccines. Whether a product has one, and how much, varies by brand. Our vaccine pages list the aluminum content of each US brand from its package insert, and the planner can limit aluminum-containing vaccines per visit. Brands and ingredients →
- Catch-up schedule
- The CDC's set of rules for a child who starts vaccines late or falls behind. It gives the minimum ages and intervals for each dose and how many doses a child of a given age needs. A series is not restarted, however long the gap. Catch-up schedule guide →
- Combination vaccine
- A single shot that protects against more than one disease, for example MMR, or a DTaP combination that also covers polio and hepatitis B. It reduces the number of injections. Combination brands →
- Dose reduction (aging out)
- When a healthy child starts a series late, the catch-up rules can require fewer doses than the on-time schedule, or none at all once the child passes a certain age. Hib and PCV work this way, as do the last DTaP and polio doses. Which vaccines can be delayed →
- Grace period (4-day)
- The CDC counts a dose as valid if it was given up to 4 days before the minimum age or minimum interval. Doses given earlier than that may not count.
- Medical exemption
- An exemption from a school vaccine requirement for a child with a medical reason, usually documented by a licensed clinician on a state form. The paperwork and who can sign it differ by state. Exemptions by state →
- Minimum age
- The youngest age at which a dose can be given and count. A dose given earlier (beyond the 4-day grace period) may need to be repeated.
- Minimum interval
- The shortest time allowed between two doses of the same vaccine. Waiting longer is always allowed and does not require restarting the series.
- Personal-belief (philosophical) exemption
- A non-medical exemption based on a parent's personal or philosophical beliefs, available in only some states. Exemptions by state →
- Primary series
- The first doses of a vaccine, which build the basic protection. Later booster doses extend it. The primary DTaP doses cannot be skipped.
- Provisional (conditional) enrollment
- A state or school rule that lets a child start school while still catching up, as long as the child has begun the required series and follows the schedule to finish it. Terms and time limits vary by state. School rules by state →
- Religious exemption
- A non-medical exemption based on religious belief, available in some states. The paperwork, renewal and any education requirement vary by state. Exemptions by state →
- Thimerosal
- A preservative added to some vaccines to keep germs from growing in them. The CDC says it has not been used in vaccines for children since 2001, except in some flu vaccines, and that thimerosal-free flu vaccines are available.
- VIS (Vaccine Information Statement)
- A CDC-maintained information sheet for each vaccine. The CDC keeps a current English version of each one online, so you can read it before the visit.
Sources
Primary sources only. Links open the publisher's own page.
- CDC General Best Practice Guidelines: Timing and Spacing of Vaccines (opens in a new tab)
- CDC Thimerosal and vaccines (opens in a new tab)
- CDC About ACIP (opens in a new tab)
- CDC Vaccine Information Statements (opens in a new tab)
- CRS Changes to CDC Vaccine Recommendations in 2025 and 2026 (opens in a new tab)
- CDC Catch-up Immunization Schedule (Table 2) (opens in a new tab)
- CDC Vaccine Excipient Summary (Pink Book, Appendix B) (opens in a new tab)
- Immunize.org State Information: Vaccine Requirements (opens in a new tab)
- AAP Recommended Childhood and Adolescent Immunization Schedule, 2026 (opens in a new tab)