According to the CDC, "A vaccine series does not need to be restarted, regardless of the time that has elapsed between doses."
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Here is the CDC's Pink Book chapter on tetanus. http://www.cdc.gov/vaccines/pubs/pinkbook/tetanus.html
I suggest you read the part called, "Secular Trends in the United States." You might be surprised by the number of cases per year, the vaccination status of those people, and the typical age range.
Here are the package inserts for all the DTaP vaccines.
Here is the catch-up vaccination schedule from the Centers for Disease Control: http://www.cdc.gov/vaccines/recs/schedules/downloads/child/catchup-schedule-pr.pdf
It's not the most user-friendly, but make sure that YOU are the one who takes charge in reading it carefully. Even at vax-friendly practices, the doctor and staff may not deal with catch-up schedules a lot and therefore may not be fully educated on them. I trusted some not-so-educated nurses at DD's clinic, and they kept misreading the catch-up schedule and trying to give her more vaxxes than she needed. For example, they didn't read the all-important end notes!
If tetanus is the only disease in that vaccine that concerns you, you MAY not need all 4-5 doses (4-5 depending on which catch-up schedule you use--more on that in a moment). I'm not sure, but hopefully your doctor can give you an honest answer. I know that the 4-5 doses are necessary to ensure a 59-89% effectiveness rate on the pertussis component of DTaP. Once upon a time, parents could just get DT without the pertussis, but that is no longer manufactured; the Powers that Be decided to take that choice away from consumers. (Nah, I'm not bitter... )
Anyway, looking at the catch-up chart, it looks like in your situation, the age that she was at the time of her first two doses is irrelevant. You just want to make sure that there's a minimum time lapse between doses. So using the chart linked above....
1. Do the 3rd dose anytime, as clearly at least four weeks will have lapsed between the 2nd and 3rd doses.
2. Wait at least 6 months before the 4th dose.
3. Here's where it gets tricky. If she's 4 years old at the time of this 4th dose, according to endnote #2 on the chart, you do not--repeat, NOT--need a 5th dose.
I can't stress that enough, because they were still trying to push a 5th dose at DD's doctor visit. (Details, people! Learn to read! )
If you're in a hurry to get these doses done, you could do a 4th dose and, if she's still 3 years old at this time, wait at least 6 months for a 5th dose. If you're not in a hurry, just wait until she's 4, even if that's more than 6 months away, and get her 4th and final dose then. Obviously, this will be a personal choice on your part.
I opted to make sure that DD got her 4th dose after age 4. But I'm super cautious about the aluminum content in the vaccine, (that and I didn't want to haul her across town for yet another doctor visit), so I chose to minimize her exposure to it while still getting her caught up for kindergarten.
DD's clinic accepted walk-ins for catch-up shots. We usually got in and out with a jab and a lollipop pretty quickly and with no office visit charge to us or the insurance company. Check with your doctor to learn the policy.
How'd I do? Is this helpful? Clear as mud?
Seriously??? Do I have to cross-reference EVERYTHING my doctor tells me?! Yeesh! I didn't know this. We got DTaP largely for the pertussis, though, so I guess it's moot for us.
You can also get the information here http://www.vaccinesafety.edu/thi-table.htm In the mercury column, the asterisk with the line underneath means 0.3 mcg mercury.