The 15 to 18-Month Window: The Doses That Finish a Series
Between 15 and 18 months several vaccine series get a last dose. Here is why the CDC schedule shows a window instead of a date, and what to track.
Published
The 15 month shots and 18 month vaccines are mostly final doses. On CDC's child and adolescent immunization schedule, DTaP dose 4 sits at 15 through 18 months, hepatitis B and polio dose 3 span 6 through 18 months, Hib and pneumococcal series finish by 15 months, and hepatitis A dose 2 falls at least 6 months after dose 1.
Key takeaways
- Eight vaccine lines can have an open box between 15 and 18 months on the CDC chart, and most of them are finishing a series rather than starting one.
- DTaP dose 4 is the one dose whose CDC band is exactly 15 through 18 months; hepatitis B and polio dose 3 can land anywhere from 6 through 18 months.
- Hepatitis A dose 2 is set by an interval, not an age: CDC's hepatitis A page says the two shots must be at least 6 months apart.
- A late dose is not a restart: the CDC catch-up schedule states that a series "does not need to be restarted, regardless of the time that has elapsed between doses."
- The 15- and 18-month visits are three months apart with nothing in between, so record which doses were deferred and when they are expected.
Search for "15 month shots" and you get results for an appointment that, on the CDC chart, is not really an appointment at all. The 15-month and 18-month well-child visits share one stretch of the schedule, and most of the doses inside it are due anywhere across that stretch rather than on a particular day. This is the part of the toddler year where series end rather than begin.
The clinician chart, which carries an "Addendum updated July 2, 2025" line, shows this by drawing bars across two or more age columns. The parent-facing version, Recommended Vaccines for Young Children, dated July 2, 2025, arranges the same information as a grid a household can read without training.
Which doses fall between 15 and 18 months?
Eight vaccine lines can have an open box somewhere in this window. Most of them are finishing something.
| Vaccine | Age band on the CDC chart | Which dose |
|---|---|---|
| DTaP | 15 through 18 months | Dose 4 of 5 |
| Haemophilus influenzae type b (Hib) | 12 through 15 months | Final dose of the series |
| Pneumococcal conjugate | 12 through 15 months | Dose 4 |
| Measles, mumps, rubella (MMR) | 12 through 15 months | Dose 1 of 2 |
| Varicella (chickenpox) | 12 through 15 months | Dose 1 of 2 |
| Hepatitis B | 6 through 18 months | Dose 3 |
| Inactivated poliovirus (IPV) | 6 through 18 months | Dose 3 |
| Hepatitis A | at least 6 months after dose 1 | Dose 2 of 2 |
Two rows behave differently from the rest. Hepatitis A dose two is set by an interval, not an age: the CDC hepatitis A vaccination page, last reviewed January 31, 2025, states that the shots "must be at least 6 months apart." And influenza returns every season from six months of age onward, so a fall visit in this window carries a flu vaccine that a spring visit does not.
The doses that opened at the first birthday, and which ones may already be done before this window starts, are laid out in what happens at the 12-month visit.
Why the schedule shows a window
Because immunity does not arrive on a birthday. The bands on the CDC chart mark the stretch in which a dose is expected to work as intended, and any date inside the band counts as on time.
That has a practical consequence households notice: the same vaccine can appear at different ages on two children's records without either being wrong. The 15 and 18-month visits are also spaced further apart than the infant appointments were, so a dose deferred at fifteen months has three months of daylight before anyone looks at the chart again. Nothing prompts you in between. The window is generous, and that generosity is exactly what lets a dose slide.
The 15 month shots are also where combination products do their heaviest lifting. A single DTaP-containing product can carry several disease names at once, so a record showing one injection may cover four or five lines — how combination vaccines read on a record unpacks that naming.
What "late" actually means here
Late is not a restart. The CDC publishes a separate catch-up immunization schedule for children and adolescents, also carrying an addendum dated July 2, 2025, and it states plainly that "a vaccine series does not need to be restarted, regardless of the time that has elapsed between doses."
That document is written for clinicians. It works in minimum intervals — the shortest gap allowed between two doses — rather than in ages, and reading it well requires knowing which product was used and on what date. A parent does not need to work it out. What a parent needs is the dated list to hand over, because the catch-up rules cannot be applied to a record nobody can produce.
Two things do change the picture and are worth raising at the desk: a dose given earlier than the minimum interval may not count, and a series interrupted by an illness or a move is still a series in progress. Both are questions for the practice, not calculations to run at home.
What to keep at home between the two visits
Three months is long enough to forget which doses were deferred. Write down four fields after every visit: the vaccine name exactly as printed, the date, the dose number when it is shown, and the product name if a combination shot was used.
Then keep one short list beside it — the doses still pending and roughly when. Hepatitis A dose two is the usual resident of that list in this window, because its due date is an interval rather than an age and no well-child visit is scheduled to catch it.
Vaxnext, the free app for iPhone and Android from the people who write this blog, is one place to keep both lists. It holds a folder per family member with no limit on how many, tracks the CDC child and adolescent schedule alongside the annual flu vaccine, and takes a custom entry when the product name on the paperwork is not in the built-in list. Its reminders are local notifications on the phone, so a hepatitis A dose due six months from now still surfaces on a phone with no account and no connection. The app is free because it carries AdMob ads, declared as such on Google Play and never shown on the record screens.
This article is general information about how the CDC schedule is laid out, not medical advice; what your child is due for is a question for your pediatrician. It covers the 15 to 18-month window only — not the 4 to 6-year school doses, not any state's school requirements, and not what to do about a reaction, which is a call to the clinic or to emergency care.
FAQ
Why does the schedule show a window rather than one date?
Because the recommendation is an age range in which a dose is expected to work as intended, not an appointment. The CDC chart draws DTaP dose four across 15 through 18 months, and hepatitis B and polio dose three across 6 through 18 months. A child vaccinated anywhere inside a band is on time, which is why two children the same age can carry different-looking records without either being behind.
What happens if the 18-month visit slips by a month?
A short delay does not normally undo anything already given. The CDC catch-up immunization schedule, updated July 2, 2025, states that "a vaccine series does not need to be restarted, regardless of the time that has elapsed between doses." The practice resumes from where the record stops, using minimum intervals rather than ages. Bring the complete dated record to the rescheduled visit so those intervals can actually be checked.