Questions About Vaccine Dosing and Administration | The Children's Hospital of Philadelphia

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Questions About Vaccine Dosing and Administration

Multiple doses of vaccines

Why are some vaccines given as a single dose and others as multiple doses?

Vaccines may be given multiple times for a few reasons:


Questions about vaccine dosing and administration

Vaccines given at the same time

Q. Can you tell me if there are any studies that discuss the risks when vaccines are given at the same time?

A. Before licensure, vaccines are always tested with other vaccines that would be given at the same time; these are called concomitant use studies.

More information

"Too Many Vaccines: What you should know":

Our book for parents, Vaccines and Your Child: Separating Fact from Fiction, published in 2011 by Columbia University Press, also addresses questions related to vaccine testing as well as multiple vaccines.

This book is available from:

Extra doses of vaccines

Q. My grandson recently got the Hib vaccine, but he had already had it. Can this have any effect on him?

A. No. An extra dose of any vaccine is similar to being exposed to the virus or bacteria in nature; the difference is that with a dose of vaccine you know that the exposure happened. If your grandson was exposed to Hib in the community, you would not necessarily know about it, but in either case, his immune system will respond in the same way — it will recognize the Hib as “foreign” and make an immune response to prevent disease.

Minimum time between doses of vaccines

Q. My infant received vaccines at his 4-month check-up. He is scheduled to receive more vaccines at his 6-month check-up, but it is scheduled for only seven weeks from his last visit. Is this safe? 

A. Vaccines on the schedule are recommended to be given two months apart; however, all vaccines also have what are known as “minimum intervals.” These are the required amounts of time that must pass between doses of vaccine. They are determined when the vaccine is developed by studying the immune response to each dose to make sure that the response from a previous dose won’t interfere with the response to the next dose. For the vaccines typically given at 4 and 6 months of age, the minimum amount of time required is four weeks, so your son’s appointment will meet this standard.

Learn more about the vaccine schedule»


Updated: February 2013

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