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:
- Some vaccines are given more than once so that a greater number of people can be protected against the disease. For example, when adults get the chickenpox vaccine, 78 of 100 will be protected after one dose, but that number rises to 99 of 100 after the second dose. Since the additional dose is not harmful and provides extra protection even in first-dose responders, everyone is recommended to get the second dose.
- Some vaccines provide a low level of protection after a single dose, but additional doses provide a greater response, so that individuals are better protected. This is true of the vaccine for Haemophilus influenzae type b or Hib.
- Another reason for multiple doses is that immune responses last longer after more than one dose of some vaccines. The pertussis vaccine is an example of this.
- Some vaccines protect against agents that change often enough that older versions would not offer protection. The influenza vaccine is an extreme example of this, requiring new versions almost every year.
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.
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- Vaccine Education Center