|From the back of the Kansas Immunization Requirements form|
Our office only gives the MMR, Varicella, and Hepatitis A vaccines on or after the birthday (except for travel exceptions as explained below) for this reason. We are also very careful to not give the second Hepatitis A vaccine if it hasnt been 6 months since the first dose. We have about 1-2 kids a year who move in from out of state and are told they got a vaccine too soon and it needs to be repeated. In Kansas there is a 4 day window before the first birthday that allows the "1 year vaccines", but some states allow for earlier vaccines and others don't have a window. I included this in a blog a couple years ago, which briefly discusses why early vaccines don't work. Since this issue comes up often enough, I'd like to discuss why some vaccines are spaced they way they are.
Vaccines are made in different ways and the body responds to them in different ways.
- Live attenuated vaccines are made from weakened virus that teaches the body to recognize the real virus but doesn't cause the symptoms of the virus. Examples of this type are the measles, mumps, rubella, and varicella. Many people respond to the first dose of these, but a second dose is given to help those who missed the response the first time. The second dose doesn't boost the first, but it gives a person a second chance at making immunity.
- Inactivated virus vaccines are made by killing the virus and using it to make the vaccine. Several doses are needed to build immunity to these. An example of this is the inactivated polio vaccine.
- Toxoid vaccines prevent diseases caused by bacteria that produce toxins in the body. The toxins are weakened into toxoids so they cannot cause illness and put into the vaccine. When the immune system receives a vaccine containing a toxoid, it learns how to fight off the natural toxin. The diphtheria and tetanus portions of the DTaP vaccine is an example of toxoid vaccine. Several shots are needed to build and continue immunity over time.
- Subunit vaccines include only parts of the virus or bacteria (subunits) in the vaccine. Because these vaccines contain only the essential antigens and not all the other molecules that make up the germ, side effects are less common. The pertussis (whooping cough) component of the DTaP vaccine is an example of a subunit vaccine.
- Conjugate vaccines fight a type of bacteria that have antigens with an outer coating of sugar-like substances called polysaccharides. This type of coating disguises the antigen, making it hard for a young child’s immature immune system to recognize it and respond to it. Conjugate vaccines are effective for these types of bacteria because they connect (or conjugate) the polysaccharides to antigens that the immune system responds to very well. This linkage helps the immature immune system react to the coating and develop an immune response. An example of this type of vaccine is the Haemophilus influenzae type B (Hib) vaccine.
- Passive immunization is a bit different than any of the above. Either catching a disease or getting any of the above vaccines stimulates your immune system to make memory cells to fight of that specific germ if it comes in contact with it. Passive immunity results when a person is given someone else’s antibodies. The protection offered by passive immunization is short-lived, usually lasting only a few weeks or months, but it helps protect right away. The Synagis (RSV) vaccine given to high risk infants is one of these vaccines.
Why are vaccines repeatedly given?
Some vaccines, such as the DTaP, need several doses to help the body develop a strong immunity against the germs, and later boosters are required to maintain that level of protection. Other vaccines, such as the MMR, require more than one dose to insure that most people develop the protection.
We're traveling Internationally and my 6 month old got an MMR that "doesn't count" -- Why?
Sometimes maternal antibodies (fighter cells from mom that got into baby during pregnancy) inhibit the body from being able to build its own antibodies well against a vaccine. (But they are good because as long as they're in the baby's body, they fight off those germs!) If a disease has a low incidence, it is acceptable to let the maternal antibodies do their job for the first year. By the first birthday most maternal antibodies have left the infant, so a vaccine can be used to build the baby's immunity. If there is a high risk of exposure, as there is for measles in many parts of the world, it is recommended to give the vaccine as early as 6 months in case the maternal antibodies are already too low for infant protection. If the levels are still high, the vaccine won't work, but the baby should still be protected against the disease from mom's antibodies. At some point the maternal antibodies go away, we just don't know when, so the baby who gets the MMR early needs another dose after his birthday to be sure he's making his own antibodies once mom's go away. The second dose can be anytime at least 28 days after the first, but we traditionally give it between 4-6 years, with the kindergarten shots. (And yes, I realize there are some measles outbreaks in the US, but the experts have not said to start giving that extra dose to babies who are staying here yet. If you are worried, talk to your doctor.)