A combination vaccine is two or more different vaccines that have been combined into a single shot. Combination vaccines reduce the number of shots, which is more comfortable for the child. They also require fewer office visits, which can save parents time and money and make it a little easier to get all the necessary vaccines according to the recommended schedule.
Combination vaccines have been used in the United States since the mid-1940s. Examples of combination vaccines in current use are: DTaP (diphtheria-tetanus-pertussis), trivalent IPV (three strains of inactivated polio vaccine), MMR (measles-mumps-rubella), DTaP-Hib, and Hib-Hep B (hepatitis B).
The available scientific data show that simultaneous vaccination with multiple vaccines has no adverse effect on the normal childhood immune system. A number of studies have been conducted to examine the effects of giving various combinations of vaccines simultaneously. These studies have shown that the recommended vaccines are as effective in combination as they are individually, and that such combinations carry no greater risk for adverse side effects. Consequently, both the Advisory Committee on Immunization Practices and the American Academy of Pediatrics recommend simultaneous administration of all routine childhood vaccines when appropriate.
Years ago there were some concerns over the addition of preservatives in combination vaccines. Of specific concern was thimerosal, a preservative that contains mercury and was used in the MMR vaccine. While there is no convincing evidence of harm caused by the low doses of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site, in July 1999 the Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure. Since 2001, with the exception of some influenza (flu) vaccines, thimerosal is not used as a preservative in routinely recommended childhood vaccines.
Additional information about vaccine ingredients can be found here.