he decision to vaccinate your children is one that should be made after researching the positives and negatives of each vaccine. How effective is it? How likely is exposure to the disease in the first place? What are the potential side effects? When choosing vaccines to have administered to our son, we spent many hours reading information and discussing before deciding how we would proceed. We selected the vaccines that we felt were important, and the order in which we wanted them administered. Unfortunately, in our concern about the possible reactions to the main ingredients, we overlooked something: the potential reaction to the vaccine's carrier agents, preservative, or other additives.

After our research, we decided that the DTaP shot was the one which we most wanted administered to our son Aidan. So after explaining our reasoning to the doctor, we had him administer the first dose.
The next day, Aidan vomited repeatedly. Concerned, I called the doctor's office, and was told that Aidan had probably picked up a stomach virus while in the office. This seemed quite likely to me--in our house, we joke that the reason for well child visits is to provide the opportunity for kids to get sick and need further doctor appointments. The vomiting continued for a second day. After that, Aidan was fine. Definitely a stomach virus, we decided.

Two months later, Aidan had a second dose of DTaP. Again, he started vomiting the next day. And continued for over a week. Stomach virus again? This seemed like a bit too much coincidence for me, especially given the more severe illness the second time. So we did further research. Everything I read about the DTaP shot said that this wasn't a reaction to the shot. I even called the CDC's vaccination hotline (1-800-CDC-SHOT) and they stated that this wasn't caused by the vaccine.

Finally, on about the fiftieth perusal of the DTaP vaccine insert, I noticed that one of the "inactive ingredients" was the preservative thimerosal. Best known as a preservative in eye care products, thimerosal was also the substance used to preserve the dead vaccine components of the DTaP shot.
I almost immediately knew I'd stumbled upon my answer! When I first began wearing contact lenses, I discovered that I had a bad reaction to thimerosal. When I researched further, I discovered that yes indeed, an allergic reaction or sensitivity to thimerosal could appear as vomiting. Needless to say, we had to discontinue the DTaP series, and now look more carefully at the "inactive ingredients" in various vaccines.

Thimerosal isn't the only vaccine additive that causes a high number of reactions. Other components--including gelatin, egg, and neomycin--can also cause problems after the vaccine has been administered. The following are among the more common ingredients. (NOTE: The following is not a complete list of minor vaccine components. If you are concerned about potential reactions to any part of a vaccine, please discuss it with your pediatrician.)

Thimerosal is a mercury-based preservative. Most commonly known for its presence in eye care products, it is also used for many dead-virus vaccines. While it usually causes only a mild irritation to eyes [redness and slight burning], as an injection it can cause nausea, vomiting, even shock in extreme cases. In 1998, the FDA banned thimerosal's use in over-the-counter drugs because "safety and efficacy have not been established for the ingredients" which are used to create it. It is, however, still present in many vaccines, including DTP, DTaP, Hib, Varicella, and IPV.

Gelatin is used as an inert stabilizer in several vaccines, including MMR and Varicella. In addition to being an animal product (and therefore probably not wanted by vegetarians), it can also, in cases of extreme (and extremely rare) allergic reaction, cause an anaphylactic reaction.

Monosodium glutamate (MSG) is added to several vaccines as a stabilizer. Adverse reactions include headaches, nausea, and vomiting. While there are no officially recorded adverse reactions that can be traced specifically to the MSG component of vaccines, anyone who is sensitive to MSG in food should watch for reactions. MSG is present in the Varicella vaccine.

Neomycin, an antibiotic, is added to vaccine cultures to prevent the growth of unwanted bacteria. Most neomycin reactions are skin irritations; however, hypersensitivity could cause a response of anaphylactic shock. Neomycin is present in the Varicella and MMR vaccines. (NOTE: other antibiotics, which may cause similar reactions, are also present in other vaccines. If any family member has a known allergy to antibiotics (including penicillin, which is no longer used in any vaccines), this should be noted before administering any dosage.)

Egg proteins are present in miniscule amounts in vaccines which are prepared using chicken embryos. While those with mild reactions to eggs are unlikely to react to their presence in vaccines, anyone with a history of anaphylactic response to egg or egg protein has a greater chance of adverse reactions. In 1998, the recommendation that people with egg sensitivities not receive the MMR shot was changed; however, the method of developing the vaccine was not, so egg proteins may still appear in any MMR dose.

Formaldehyde--yes, the same formaldehyde used to preserve pigs and other animals for biology class--has the same basic use here as well: it inactivates or kills unwanted viruses that might be found in the cultures used to produce vaccines, and acts as a preservative for dead virus vaccines. It is found in some formulations of Hib.

Sulfite is used as a stabilizer in several vaccines; its main purpose is to prevent the vaccine from being altered by changes in environment. Sulfite is also found in many foods and alcoholic beverages, and is often an irritant for people who suffer migraines. It can also cause diarrhea, vomiting, and severe cramps. Sulfites are present in some flu vaccines.

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Noël-Marie Taylor is a freelance writer located in Columbia, Maryland. Her work has appeared in many magazines, including PC Magazine and The Mother Is Me. A stay-at-home mom to two children, she is also the designer of several cross-stitch kits for children.