The oral version of the polio vaccine is being eliminated because of the risk of a rare reaction. "The oral vaccination of polio uses a live vaccine that in one in a million cases turns into a serious from of the disease," Dr. Fischer says.

Children now receive an inactivated polio virus -- IPV -- which does not have this risk but has the same results.

Common reactions to the other vaccinations include a low fever or slight swelling in the area. Dr. Fischer says serious allergic reactions to vaccinations are uncommon. If parents are concerned they should look for a skin rash or hives.

Boker, who is also a nurse, said her 10-year-old son had a bad reaction to the MMR vaccine. "His upper shoulder was red, swollen and you could feel the heat coming from the infected area," Boker says.

She gave her son Tylenol for the muscle pain and made him do light exercises, but she did not allow him go to Little League practice.

All 50 states also allow medical exemptions to children who are immunocompromised, allergic to vaccine constituents or who have certain illnesses. The parent must have a doctor document the condition. In 48 states exemptions based on religion or faith are permitted. Philosophical or personal reasons in 15 states constitute exemptions.

Dr. Fischer says that because of legislation, not all the usual childhood vaccines are mandatory. "Parents should ask their doctors what vaccines are recommended," she says. "Legislation shouldn't decide a child's health."